At OCS, you were given the opportunity to buy from the Marine Shop or the MCX for your uniforms. Honestly, at OCS, I was hiding a broken foot and the Marine Shop was on the side of the auditorium closest to me and I didn't feel like walking over to the other side. I wish I had just gone with MCX because I had lost so much weight at TBS and my Service Alphas are forever the wrong size. I would honestly just use MCX for all your itchy uniform needs. GySgt Franco is the POC for the TBS store you will likely use. If you have questions, he is extremely helpful and can be contacted at (703) 432-6452.
On Camp Barrett, there is the "little PX" that has a uniform shop below. I would try to order your uniforms ASAP. You won't need them right away, but some commanders order inspections mid-way through the cycle.
The PX itself has most of the supplies you need for TBS, but if you don't find them there, you can go to the MCX on mainside. One thing that was random, the uniform shop below has tarps, and I haven't seen them at the MCX. Trust me, I lost mine on night land navigation :)
The uniform shop at TBS has camis, but I'm not sure if they have desert camis. THEY DO NOT HAVE BOOTS. You'll have to get these on mainside. They have EVERYTHING for Service and Dress uniforms.