I've updated this guide to be more spoiler-safe for any new players coming here.  Each sticker will be in its own spoiler section, which will be labeled according to which gym leader fight it is available for.  If you're new to the game and want to avoid spoilers, I suggest you don't open them until you're just about to fight the gym leader listed.  For example: if you just beat the 2nd gym leader and open the "available before the 3rd gym leader" section, you'll get spoiled on what happens in b