Well, the sleeve-less redshirt/yellow closures and back pants would seem to be the most obvious choice. You could easily order that off an online martial arts supply store.
....or you could amke it; it'd be much cheaper.
I think it would be cute to see Ranma's little blue outfit with the yellow shirt and cap. In the show, it looks so cheesy, but for cosplaying purposes, it'd be very much cute. ^^