LM7808 is a very old part design, it's just a higher voltage version of the extremely common LM7805 voltage regulator that is used in half of all electronic devices ever made! I got a pair on ebay for a couple bucks (including shipping), you should be able to get one from any electronics parts distributor such as digi-key.
The heat sink doesn't need to be anything more than a piece of flat aluminum about an inch square...if that....
(edit:
digikey has a couple thousand of the LM7808 devices in stock)