I've been trying to bind for around a year now, but I can never do it perfectly, and can still be seen as female. I'm 14, and I have a larger chest, so I can't bind well. I don't have an actual binder, but I might get one soon. I don't know what size I am, because I've been wearing exercise bras for a long time, even before I came out as trans, or even realized that I was. I've been wearing a soft exercise bra, with a tougher one worn backwards over top of it. It doesn't work as well as I'd want it to. I don't want to use an ace bandage or duct tape, because I know it can damage your ribs and such. Any advice?
All I can tell you is try to wait it out until you can get a real binder. But please be careful when buying one! DO NOT get one that is too small. This can cause serious damage. You should be able to inhale completely and comfortably in your binder. Also, buy one with binding material only in the front. Don't get one that's the same material the whole way around. I recommend buying a binder from Underworks. Their binders are pretty good. And on the Ace bandages and duct tape - binding with these things is really, really dangerous! Never use them to bind. Ever. Your safety comes before your appearance.
Getting your hands on a real binder is probably going to be your best bet. I know that JC2B goes up to a 5x, so if you have a larger chest you may have good luck there.
I'm large chested and I use a gc2b binder, it works fairly well, not perfect (but then again I bind on the loose side to be safe). I'm not sure how it compares to underworks though, cause I was way smaller when I had one of those. Both brands are really good and on the cheaper side. Definitely don't try to bind without a real binder, and be wary of too tight sports bras--the elastic can be just as bad on your ribs as improper binding.
You're definitely right not to use ace bandages or duct tape, it's really dangerous and really painful! I would recommend getting a real binder, it's honestly the only way I've been able to get satisfying results. I am larger-chested and exclusively use underworks binders. They're really good, but I find it's been better for me to pair them with shirts that aren't too baggy or very tight. I'm not perfectly flat even with my binder on, but with the right fitting clothes I look flat.