Where can I purchase my retainer and can my orthodontist just stop my treatment because I don't have the money to pay?

I've have my braces for over a year now. I was originally recommended to only have them for a year but I've moved about twice. I am currently behind in my orthodontist bill because I was let go from my job, it's been almost 3 months now and they will not treat my because because I owe money.I coukd get my braces off now if I wanted to, I was told my teeth ate already straight , I just have a small overbit. My retainer cost 400 dollars, do I really have to absolutely purchase it from this orthodontist