A hair doctor? physician? And how much does such a prescription cost? Does it work?

I'd talk to physican. Depends on your co-pay with your insurance company as to cost. Never used it. Ask physician if it works.

any general practitioner would be able to write a Rx for this.

A friend of mine was on Propecia for a year and then when he got off it almost all his hair fell out, it was pretty bad. His doctor told him it wasn't going to happen but i guess that big kickback he got from the manufacturer of Propecia made him not really give a $%^#..but while your on it your hair will definitely get thicker. It messes with your bodies natural DHT levels, don't say i didn't warn you. IMHO There are better alternatives.