Raiding can't always be the only answer to get supplies. Maybe getting supplies from a Hellforge but that can be expensive. A Warband should want to stay relatively hidden to not get a Chapter chasing them with or without Imperial detachments to grow and getting stronger. I know its traitor legion dependent but they still need to worry about logistics (even World-Eaters).
The logistics are crazy. First check out this page; scroll a bit into the chat and you'll come to Frater Dominus' post. Frater Dominus is actually Aaron Dembsky-Bowdon, renown Black Library Author who is wildly respected as the King of Renegade Astartes Fluff. It doesn't get much more authoritative than that!
In essence it breaks down a couple different ways, leaving aside raiding Imperial facilities and doing deals with Hellforges. I've listed them here in no particular order
Doing a deal with "friendly" warbands. Maybe you're all from the same legion, maybe you're old allies from a Black Crusade, maybe you hate each other but feth it, you need supplies. So you do a deal. "I'll swap you 30 slaves and a thousand lascannon power packs for your 40,000 rounds of bolter ammo and spare tracks for a land raider." So you steal what you can from raids on imperial outposts/other chaos warbands, and then swap extra stuff for what you didn't steal. Or maybe your overarching warlord (Abaddon the Despoiler, say) commands other contingents to give you weapons/ammo/food because he wants you to be efficient tools. or maybe an allied warband gives you XYZ things because THEY want you to take the risks of attacking position Alpha, and you're willing to take that risk because you need XYZ SO BADLY.
You have tributaries. Maybe it's a world that was friendly to your Chapter before you turned, so you put on the old colors and collect the old tithes because its easier than starting a fight and since the Inquisition purged your records and astropathy is hit-and-miss these idiots never got the memo that you're renegades. Or maybe you have worlds in the Eye/Malestrom that owe you fealty. They make boltrounds/harvest promethium for your flamers or whatever and in exchange you keep them from being ground into extinction by some OTHER chaos warband. "nice city there, be a shame if someone nuked it from orbit" is a POWERFUL motivator.
You own a world of your own. You're a fething Chaos Marine Warband! There's 110 of you! You're the most horrific warriors to ever exist so of COURSE you conquered a place. Raised up an army of mortals to be your lackeys/cannon fodder/guard it when you're gone. Said world is rich in certain resources, so you do a deal with a Hellforge to trade its mineral wealth for weapons. Or maybe you're also sheltering some Hereteks/have some heretek slaves who produce more-or-less all of what you need. Or maybe it's a pretty populous planet that has no problem keeping 110 marines and and old cruiser supplied.
You have a construction ship. Maybe you captured a mechanicum forge-vessel from somewhere. They're pretty widespread and provide a wealth of materiel for campaigns. It's great! Can turn all sorts of raw materials into weapons and sundry other supplies. You only need the raw materials. So you occasionally set up shop on mineral-rich asteroids or do deals with sketchy rouge traders or xenos who don't care you're chaos worshippers as long as the price is right. What price do you pay? Maybe a percentage of the finished goods. Maybe you attack a rival for them. Maybe the price is that you don't turn their planet into a hellscape covered in corpses.
You've done a deal with Devils. The warp is a WILD place. The Eye of Terror is full of demon-worlds that do nothing but craft swords for khorne's legions, or distill human souls into fine wine, or anything else you can think of. Maybe your warband's sorcerer has made a Pact with one of the Demonic lords that rule such worlds. You offer it your warband's worship, or a specific amount of sacrifices, or the purging of a certain world or anything else demons pact with mortals for. In exchange, the Demon's worlds provide you with bolters that never run out of ammunition, or a ritual that turns its victims into armor panels that can repair your tanks/ships, or a creepy cornucopia that endlessly supplies promethium. The possibilities for this one are literally limitless.... as long as the Demon lord is appeased...
Raiding probably is the biggest source of resources for warbands(at least the Night Lords trilogy suggests so). There are a few things you need to take into account
- Space ships are big and largely self-sufficient(some resources such as fuel and spare parts can run out, but in terms of things like food and water, they can produce/recycle their own). Warp capable ships are massive, and expected to hold enough supplies to last months or years between resupply(since there's always the chance of getting lost in the warp, and travel generally taking a long time)
- The imperium is massive, and the distances are long. If they attack, say, a remote supply depot, it may well be weeks before anyone even notices, and months before it's investigated. By the time anyone notices, the trail will likely have grown cold.
- Related to the above, there just aren't that many loyalist space marine chapters. If there are a million worlds and a thousand chapters of space marines, there just isn't enough to chase after every raid, whether traitor marine, other chaos force, or some foul xenos.
- In some cases, they may even want to be chased by loyalist space marines: After all, any space marines chasing after them will have many supplies that are hard to find elsewhere, such as astartes-pattern power armor, bolters and other supplies specific to space marines(and of course, geneseed).
- Time flows differently in the warp and eye of terror. What has been 10 millenia for the Imperium, has in some cases been only centuries for a particular warband. So in some cases, there's no need for raids that would be frequent from the perspective of someone in the real space.
Combine the above with alternatives to raiding as suggested by Dario Quint in the accepted answer, and it's quite easy to see that warbands can maintain adequate supply status without a steady base(although having one certainly helps massively).
Chaos Warbands usually have a home planet / base of operation (emphasis by me):
Oliensis - Oliensis is a Daemon World located in the Eye of Terror that is devoted to Slaanesh, the Prince of Pleasure. The entire planet is actually a colossal morbidly obese man curled into a fetal position. The surface is home to hordes of Slaaneshi daemons and also serves as a base for Noise Marines of various Slaanesh-dedicated Chaos Space Marine warbands.
From the Daemon World article on Fandom
So, they can use their base of operation to produce what they need for their raids. But with the capture treasures of a raid, they can pay the Dark Mechanicum for the other goods they can't produce themselves.
And for the Warband not to be chased by a Loyalist Chapter is the reason for them to often have their base of operations within parts of the galaxy not accessible to the Imperium, e.g. the Eye of Terror or Maelstrom.