Jasco (online and in catalogues) sells both pants and tops that they advertise as "Can't See Through 'Em"
I bought the pants for graduation and am quite pleased. The fabric is heavy enough and woven well enough that you definitely are not seeing through them, and they are very comfortable to wear; I wear them for work. I also have two Cherokee white tops: one with embroidery all over the front that you can't see through at all; the other is a thinner white that is still less likely to show, but I wear a tan bra with it to be on the safe side