I'm sorry to hear of your lack of support from your family; and I suspect your age puts you in a difficult spot where you don't have very good financial resources or even ability to have a paypal or credit-card account, to buy yourself the needed clothes discreetly, online. If I'm wrong (hopefully I am), maybe you can give us an idea of what you could afford, and some measurements, and collect some recommendations here. All of my own clothes were always bought for me by the women in my life until nearly age 60 when gyne started its intrusion into my life, and I felt like I had to handle it alone.
Bras are generally not cheap (although there are exceptions), and unless you can try on quite a few at a lingerie store, it will take quite a few purchased online (and a lot of research) before you finally land on ones that fit you right and are comfortable. I understand many (if not most) lingerie shops will make appointments for males before or after hours so they don't freak out the women. I haven't gotten my nerve up to try it yet myself. I started with really cheap Chinese sports bras I bought online; and although they're not my favorite, at least they prevent the pain I was getting when riding bike, and I still use them for that, and there's no tell-tale bra outline clearly visible through my jersey. The problem I've had with them for anything else is that they bite into the fronts of my shoulders. It's more comfortable (but more expensive too) to have something more resembling a bra, although I'm still cheap, and I want something that hides well, too. (With reasonable shirts, ie, not tight, my gyne is still hidable, and hopefully will remain that way. My mother's breasts are very small, which I hope will work in my favor.) Running without a bra is awful. My need is greater now than it was last summer though, so I'm wondering how I'll get through the coming summer discreetly. Going braless ranges from irritating to painful. I hate it!
There have been a couple of other 15-year-old boys on the forum recently, but they don't stick around long. It would be great if you could all communicate and see what has helped the others, in the various aspects, social, family, fitting, finances, etc.. I really feel for you. Kids can be so cruel. For me as an introvert, it was bad enough in high school even without having gyne yet. Hopefully this forum will turn out to be a huge help for you.