i get it off the darknet only because of convenience and i already have bitcoin there. i wouldn't sign up to a darknet market just to get finasteride though, it's available easily on the clearnet as it's not a scheduled drug
i used to use inhouse, before that a UK online pharmacy.
i've never gotten a prescription for it legally, cba. i doubt my doctor would even script it, he's bald. he's the type to say "don't use finasteride, i prescribe to shave it, grow a goatee, hit the gym, get a tan...."