Your closest Army, Navy, or Air Force base will suffice. Hell, even Coast Guard might work; I'm not sure. The transpo office at any of those can get your paperwork started. However, this is all contingent upon you having both your orders to Ft. Sam and orders to USUHS, and getting on base.
Having just come back from a 2 day trip to Fort McCoy to get my uniforms I strongly second this.
It turns out my Army HPSP recruiter was totally wrong about getting commissioned, (invalid since I don't have the appointment yet) and about going to my nearest post to get uniforms, (before I am in the DEER system, so I can't get ID).
I spent the first 2 hours at the security office trying to get a vehicle visitor pass, only got in once they called the CPT that commissioned me, (incorrectly) who vouched that I was a commissioned officer, (which I am not yet).
I did learn a few valuable things however.
1. Its worth trying on the uniforms in person, the sizes that fit me best were not what I expected.
2. Buying the stuff you need for OBLC takes longer than I expected, I spent a good 5-6 hours before I had the entire list purchased. So if you can, buy your uniforms ahead of time so you don't have to rush at Ft. Sam.
2. The folks at the Post Exchange are super helpful with getting the right stuff, ( the women at the alteration store pointed out that my ASU was not only the wrong size but I had grabbed the wrong jacket). I highly recommend seeking their counsel when buying your uniforms.
3. Don't bother going to any Post or Base until you are in the DEER system, unless you want to get an invalid commissioning, (wouldn't have gotten on post without it ironically enough) and spend a good many hours trying to explain why you don't/can't get your military ID.
4. Most importantly, those uniforms are WICKED SWEET! Heck it might be worth joining the Army just to be able to wear an ASU at parties, (a HUGE hit with the ladies I imagine
😀