I always used brown paper bags and either decorated them markers, stickers or I even put wrapping paper on them but I can see how some kids might think that's not the "cool" think to do.
You can also make covers out of scrap cloth or even old tee shirts! I have a pattern for that in one of my "uses for old tee shirts" books. It's cool if you use a really neat tee shirt that's worn. You make it just the same as you would with a paper bag except you secure it with a glue gun.
If I were you I'd buy whatever at the beginning of the year. If it gets ruined during the year the boys have to either replace them with their own money or use brown paper bags. This might make them be a little more responsible about them!