How do I get my mom to get me a chest binder without her knowing what it's actually for?

I wish I had an answer to this mate, if I did I'd be doing it. The most I can suggest is a sports bra and maybe some baggy shirts/hoodies.

it depends on your age and how in charge you are of your finances

I'm not sure how you can, but i know you can bind with sports bras as long as they are nit too bad