Takeoffs from someone upgrading the cams in their PI car. Keep your eyes on the classifieds here and the other various Mustang & 4.6L boards. You could try the junkyard too but they'll probably want to sell the complete heads and not separate the cams.
If you buy them new from the Ford dealership, you'll be getting close to the price of a set of aftermarket cams.
I bought some brand new cams from evil-bay for 50$ shipped. They were supposeably take offs from a new PI head which got aftermarket cams. Now if only I can get the chance to install them.