fast & cheap would be the USPS Priority Mail stickers.
High cost, but high quality would be the wide range of 3M sheets.

recently, someone in philly has been tagging little sticker flowers advertising sticker production and qualtity "stickness" lol i'll make a note of the website or name next time i'm out and about.

however, i dont really know how smart or original it is to be tagging with the exact same image all over. i love the guys that have the same character, but simply vary it a bit with outfits, expressions, etc. It's much more artisitc and helps you learn the persons mood at the time and ever changing look.