You can purchase a birthday cake at many of the restaurants if you are eating there. We got a good one (very tasty, not that expensive, maybe only$10) at LTT for lunch once. They decorated the table with pixie dust and little streamers.
We also got the very expensive one at GF (shaped like Mickey's head) and it tasted horrid (very dry, not good texture, not worth the money).
Like starbox said, each of the parks have a birthday button that you can pick up for free at Guest Service locations inside the parks - Town Hall at MK, GS at Epcot, GS at AK and GS at Disney Studios. We also got one from Security at the International Gateway at Epcot once.