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I use StickerMule for work, excellent quality and good pricing. Better to have the design in a .psd or .ai file to make it easy, but they can usually convert it for a reasonable fee. Lots of great companies out there but I only have experience with SM.
my school had a maker lab with a plotter. i used to go to an arts and crafts store to pick up rolls of vinyl and transfer tape ($20-$30) and it would make like 100 small stickers. yours might have one too, many art classes have them as well.
if you intend on making lots and lots of stickers even in the future, you can pick up a used cricut from ebay, craigslist,offer up, kijiji for less than $100. then you can make whatever you want, whenever you want. you could even sell them or do custom sign work. its a skill many dont know, and your parents might be willing to invest in it for you to learn.
getting good quality vinyl (die cuts, some people call them) from internet sites can be pretty expensive, and they are limiting in what you can cut out from them, at least easily anyways.