There is a good one, and lots of other good information, at earlenescakes.com She provides a template you can use for your own.
I suggest taking a look in the business forums. There have been multiple post about wedding cake contracts and some people have even posted their own for a reference.
If you do a search of the forums I suggest using "wedding contract" as your search terms
you can buy one off of legalzoom.com - not expensive, but they have a bakery one you can use and change to match your needs.
had to edit - had website address wrong - it is right now!
Mine is posted on www.cakeboss.com
Thank you for all your responses! I really appreciate it!