Hey, just wanted to let you know that I was once in your position (in HS with no real binder and no way to buy one). Here are a few things I did:
-Tight stretchy undershirt. Champion, Underarmor etc, available at Target/WalMart/Etc. I sometimes see these in the women's section for working out, but on a hangar they just look like t-shirts. They are stretchy (lycra?) and breathe really well.
-My first binder was (no kidding) a girdle with the old hook and eye closure (how a bra is closed), with a few of the middle hooks undone. This worked for a couple of years.
-If you can buy on eBay, you can get a GC2 compression shirt, which (for me) work better than underworks because they are less stiff and don't compress the tummy as much. About $30 including shipping.
-If you have money, but no access to a card, get a pre-paid debit card that you can buy with cash (GreenDot, UpsideCard, tons of these out there).
-Try hiding them with button-down shirts (tshirt, tank, or binder underneath).
Also check the binding forum.
