There are many great free tile sets for games available on the internet.
Just some examples for cool resources:
Many of these tilesets use one solid background color instead of transparent color value. It's a quite time consuming procedure removing them from the set - especially if the set already comes in single images.
TexturePacker helps you with this problem with the new feature called Heuristic mask. Just add the single sprites and activate it:
I think this new feature will make it much simpler to work with these great assets.