Have you tried Pantothenic Acid (B5)? I was getting acne outbreaks while on Propecia and it cleared it up right away. Maybe you could try finding a different dermatologist, there are a lot out there. It could be that she isn't prescribing it to you for your own health, did she do any studies?
If you are SURE it will be safe for you to take it then just order it online. If you do a search you can find it relatively cheap.