Cakes like this are so much fun to do but do require quite a bit of lead time. Expect most bakeries to have a minimum notice for such cakes, depending on the type of cake and the bakery's current bookings. I require a 2 week lead time or even more for elaborate cakes. Simpler cakes can sometimes be done the same week but expect to pay a rush fee if the bakery already has a heavy schedule on the books. Most custom bakeries also require a minimum order (as I said in a previous post, Duff from Charm City Cakes has a $1,000 minimum) so that will mean that no matter how small and "simple" the cake is, you will be paying at least the dollar amount of the minimum established by the designer.
The bottom line is, have fun with the theme of your event, try to use
something personal about the honoree to use in the design of the cake and give your baker enough time to come up with a fabulously executed design for your event.