Sticker Buying Guide!
(now sorted by usefulness)

http://stickerapp.com/
Best for Value
Variety: Pre-made, upload your own
Type: Single sticker (mass print), sheet, die cut, kiss cut, basic shapes
Cost: $0.12 - $0.80 USD per sticker per inch of a single design, price per inch goes up for bigger stickers, but decreases for bulk buys. ~$0.40 USD per inch sheet
Quality: Let me know!
They have some 'teen sk8r' stickers, and a custom sticker uploader.

http://www.stickeryou.com/
Variety: Upload your own
Type: Single sticker, sheet, die cut, roll, static cling, wine bottle lables, business cards, temporary tatoos
Cost: Varies, depending on type, sticker sheets are $10 USD per page and have discounts for bulk buys
Quality: Let me know!
They make original stickers too, but the true virtue of stickeryou is the mass variety of ways you can have your sticker printed. I haven't even browsed every possible option. The sticker arrangement tool does use flash player, but the rest of the website does not.

https://www.redbubble.com/shop/stickers
Variety: Everything, community uploaded
Type: Single Stickers, die cut
Cost: ~$1 USD per inch
Quality: Varies, community uploaded
For the lazy, redbubble has everything, but you're going to be paying a premium. Keep 'sticker' in the search URL to just search stickers.

http://www.unixstickers.com/stickers
Variety: Unix, programming, 'geek' 'culture', memes
Type: Single Sticker, die cut
Cost: ~$1 USD per inch, discount on bul buys
Quality: Let me know!
This is where you get those Linux distro stickers from.