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...
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.