how can I get a silicone mold made for a cake?

I don't know where you can get a mold made but I have heard that they are extremely expensive to have made. Try this website, if you read one of the testimonials, the artist made a caketopper using the lincoln memorial and might be a good alternative for you. Hope this helps!

http://www.carolsakai.com/weddingtesti.html

I don't know that it's cost effective to do a one-off mold out of silicon. I'd make the cake in the general shape and add the details using fondant icing.

If not http://www.soapandcandlemolds.com/custommolds.asp does custom candy and soap molds. Maybe they can scale up?

Some times you can make them otherwise try those homware shops like housewarehouse or ikea they have tones of different silicon pieces. Maybe if you did your cake in all different silicon molds and then stuck them together with glased icing. It would be easier and cheaper.