This is for one of my friends, but also anyone who needs it. If you're cosigning buying a binder, but you don't think you can with your parents, I will provide tips, advice and links that I found on the internet.
One thing that I see a lot is to wear a sports bra or two (when I say two I mean double them up)
Another one is finding someone that you trust in person, like a friend, teacher, or anyone else who does own a binder, and ask to try it and see if it fits (this one would be kinda hard to do tho, so don't go looking for someone). You should be able to breathe easily, bend down, there should be no pain, but make sure that it actually binds, if it's too big it will do nothing. NEVER bind for more than 8 hours a day, take it off if it gets uncomfortable, starts hurting, gets itchy, or anything else feels off. Don't sleep, swim, exercise, or shower/bathe in you binder.
Links:
https://www.quora.com/How-can-I-get-a-binder-with-no-money-and-without-my-parents-knowing
https://www.reddit.com/r/ftm/comments/4zw4tf/how_to_purchase_a_binder_without_your_parents/
https://lgbtchat.net/threads/how-do-u-get-a-binder-without-parents-knowing.74691/
(I am not a professional, I recommend that you really do look it up, but if your parents go through your search history this could be a last-resort.)