To start, you will need to go to your nearest military base (any branch) to the Military Clothing Sales store, which is part of the Exchange. Army installations have the largest selection of Army uniforms (no surprises there) but other branches carry a limited selection. However, by a stroke of luck I was able to buy my entire uniform from an Air Force Base. Even if your closest Base isn't well stocked with Army clothing, you can at least buy the items that don't require super specific sizing (socks, undershirts, cap, etc.) and then buy the jacket and trousers online. Some of my friends had to buy online and return what didn't fit and try again online. It just comes down to which branches bases are closest to you and how well they are stocked.
The total cost for all these is under $400, variation is due to sizes and boot style choice. Luckily, you will get $400 reimbursement at BOLC for your first uniform.
You may have heard a rumor that HPSP students can get up to $600/year in uniform expenses reimbursement. When I asked my recruiting office about this, they said that is only for enlisted servicemembers, not officers. If I later find a different answer, I'll update this post.
Happy shopping! I had a lot of fun shopping for all the pieces and felt so proud when I finally was wearing a proper Army uniform!