Looking for Pokémon t-shirts can be a nightmare, since all of the official ones have the logo plastered all over it (as well as Nintendo's, 4Kids and maybe Warner Bros. if they can get away with it). The shirts seem to be a method of advertising rather than a way of expressing your love for it.
Fortunately, on my travels through Google, I have. (And what's coming up is a few pictures of me in the t-shirts I bought. Shield your eyes...)