Look for ones that say they have a sliding scale. They will talk to you at the first appointment about your finances.

But, honestly, Perna is right. Getting therapy for free can complicate things. The fee helpd create a balance of power. You seem to just want it for as cheap as possible, but I'd definitely recommend paying as much as you can even if it's $20-30. Therapists train and work hard to be able to offer their service.