ISO Custom Birthday Cakes
Depending on how precisely lobsterlike you want it, you could try Noe Valley Bakery. Cakes are perfectly adequate and they've done custom size and decorations for us in the past. (Not sure they would have a lobster cake form, though.)
What's your budget for "reasonable"?
Can't get links to work -- here's their site URL: http://www.noevalleybakery.com/
If anyone could make a lobster-shaped cake, it would be Kelly at Inticing Creations. She makes some really beautiful, unusual cakes that actually taste good, too. I know a pastry chef who asked her to make his wedding cake, and he was quite pleased.
She makes whimsical cakes, too.