Up through the late 1980's, Kraven's story was pretty simple.
In his first appearance, he describes his abilities as follows:
Now, the most obvious way to interpret his statement is to assume that the potion was a one-time thing. However, every time we see him at rest in this issue, he's got a bottle or glass of some steaming drink:
While that could just be Kraven's coffee thermos, it certainly seems possible that it's the potion he refers to.
In his next solo appearance, in Amazing Spider-Man (1963) # 34, this question seems to be settled:
As he says here:
Only I know of my special jungle herbs ... my wondrous potions of power!
They give me the strength of the mightiest jungle beasts ... the strength which can match Spider-Man's own!
Note that, in every scene in his first appearance, he's with the Chameleon. Future stories would seem to indicate that they were not actually equals. Kraven may have been fudging the truth around the Chameleon, to avoid revealing his secrets.
I'll also note that this is the last Kraven appearance drawn by Steve Ditko. I don't believe other artists have necessarily chosen to portray him drinking his potion. We see little if any sign of it in his appearances over the next few decades.
Kraven continued to fight Spider-Man (and other characters) for a long time. However, in late 1987, Marvel published a 6-issue story through all three Spider-Man comics (Amazing, Spectacular, and Web of), ultimately titled "Kraven's Last Hunt". Kraven beleives he's finally killed Spider-Man, and tried to take on his identity. Spider-Man is not dead, and Kraven chooses to end his own life at the end of the story.
This being comics, that's not the end of the story.
First, in the new Spider-Man titled created for him, Todd McFarlane (co-creator of Venom, and the creator of Spawn) tells us a story about a lover of Kraven's, who claims to have created the potion for him: Calypso. While she had appeared in two previous stories, and probably was Kraven's lover at the time, she didn't seem to have a personal hatred of Spider-Man; she mostly seemed to use Spidey as a goad to egg Kraven on to doing things. However, in Spider-Man (1990) #1-5, in a story titled "Torment", she displays mystical abilities of her own. I don't recommend actually trying to read "Torment" - it's not pleasant (not due to it being a horror story; rather, due to it being McFarlane's first attempt to write a story).
She appears a few times after that, again displaying mystical abilities far exceeding any capabilities Kraven had ever shown. In spite of that, it's not clear that she is telling the truth about being the original source for Kraven's potion; it seems more likely that she was his student, not his teacher.
We do learn that Kraven had a wife and several children, some of whom choose to follow in his footsteps. Vladimir Kravinoff becomes the Grim Hunter, and tries to kill Spider-Man. Another son, Alyosha Kravinoff, initially clashed with Spider-Man, but eventually became a somewhat heroic character, before cashing in to become a reality TV star. He also killed Calypso.
However, Kraven's wife and daughter, Sasha and Ana, were responsible for bringing the original Kraven back to life. He came back as a psychopath, and claimed to be cursed and unable to die.
Spells and potions are mentioned as the means by which his family brought him back to life, but nothing is said about whether he still needs to take potions to gain superhuman abilities. It may be possible that, since his resurrection, he no longer needs to take the potion regularly; he may have gained his potion abilities permanently in the process. However, there's no real evidence pro or con that I've found.
In his most recent appearances, he has been portrayed differently for different stories. In particular, in his appearances in Unbeatable Squirrel Girl, he seems to have been convinced first to hunt monsters (like Giganto, the whale-like monster Namor once unleashed on the surface world), and, later, decides he will "hunt the hunters". In these stories, it would seem like he's turned over a new leaf.
However, we do see him elsewhere, such as in Old Man Logan, where he decides to kill Logan so he can die in battle, since Logan seems to be deteriorating. Logan, understandably, takes exception to this.
To the best of my recollection, none of these recent stories mention his potions at all.
To address your specific points:
I do not believe that it's ever been established just where the potion originated from, or how long ago Kraven took it. Calypso may have been the witch doctor - if we believe her story. She may have been someone Kraven eventually trusted to help him make the potion, and not been its actual creator.
From his early comments and appearances, I would guess he had to take the potion regularly - possibly daily.
His comments in Amazing Spider-Man #34 would tend to indicate it was simply an herbal potion. However, his resurrection seems to imply that Kraven had some knowledge that delved into the mystical. It's unclear if that knowledge was a part of the potion.
I'm not aware of any issues that clarifies where the jungle herbs came from, or whether they only exist in one particular location, or can be found in multiple jungle throughout the world.
I will note that we've seen Kraven in the Savage Land on more than one occasion. Given the relatively unique nature of the flora that can be found there, and the fact that there's both alien involvement, and old tribal civilizations about, it's not impossible that this could be the source of the herbs.
In fact, we also know that the Savage Land is one of two sources of a metal called vibranium (although the Savage Land and the Wakandan versions of this metal seem to have radically different properties). We also know of a specific, heart-shaped jungle herb that the Panther uses to enhance his abilities. Perhaps there's a connection there - a Savage Land herb that has similar effects to the heart-shaped herb the Panther uses, but isn't guarded by a country like Wakanda.
Note, however, that this is all totally speculative; I have no story references that would support these theories. And, this again supposes that Kraven is deliberately misleading his old "friend" the Chameleon in his first appearance.
The main source of the conflicting information seems to be Calypso, and her stated part in Kraven's past is definitely a retcon.
It's not entirely clear that she was telling the truth, since when she starts talking about her role in things, Kraven wasn't around to confirm or deny. She doesn't seem to have any mystical powers in her first appearance, and it seems reasonable to believe that she's constructed her own reality after Kraven's death, and increased her significance to him. She may even believe it, by that point.