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 - especiall if the set already comes in single images.
The 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.