The white shirts are now just the same shirts worn as part of ASU's. They can be bought at the PX. Trousers you'll have to go to WB4 and they will work to either DX or tailor your current sets if they are not fitting right or set you up to buy replacements.
Pants I never had to worry about. I got extras as hand me downs from a teammate that graduated when I lost my good ones. Shirts I had to buy new ones occasionally but that was more because I ruined them in one escapade or another that the white was never going to look good again.