In thinking where stickers come from, once upon a time, every pro skater, every demo, every time someone would go somewhere, there would be a million stickers, but nowdays there doesn't seem to be many at all in those common places.
Besides skate shops and promo piles to give to kids, most comps, events or anything else usually still have an abundance of stickers, but I don't think there is anywhere near as much of that going round now as there used to be.
Also if you look up "sticker bomb" or similar things, there are a lot of sticker lots from China or ebay with fake stickers from all the main brands for very little. True you are not helping anyone by buying those, but it is a good way for a beginner to get a ton of cheap stickers.