From what I've heard, the over-the-counter progesterone stuff isn't supposed to be strong enough to help. Also, it can be a little tricky/dangerous to use progesterone and lots of articles that I've read online say to only use with a doctor's supervision.
My doctor is willing to put me on the progesterone supplements as soon as a I get a BFP. I asked why he can't give it to me before then and he said it could hurt my body's natural ability to make it.
Not sure that info is of any help, but if I'm having progesterone production issues, I do NOT want hamper my body at all in making as much as it can.