First place I'd try is someone who makes wood counters. I know of a local place and here is how I found them:

My old neighbor had the most gorgeous walnut 10' island in her kitchen. I got the name of the company from her. They were about 2 hours from where we lived. But I called him when we were moving and considering walnut for our island--he actually gave me the name of a local kitchen place that was a local rep for his product in this area.

This guy made gorgeous bars tops for bars, different wood counters, etc.

I'm sure someone like this could easily make you a custom cutting board.

So perhaps you can ask around and get names of local businesses that specialize in custom wood counters.

I know, not much help.

check out http://theboardsmith.com/ This is what I'm planning on getting. He gets great reviews from folks on KitchenKnifeForums.com. He has standard sizes but does anything custom you want.

Here is a link that might be useful: The BoardSMITH

I would second bee's idea to look for someone locally, even someone who makes cutting boards. We decided we wanted a custom one for our island after about a year in our new kitchen, and I had a really nice portable one that was made by someone who lives in our state (his name was on the back). I googled him and found a website so I emailed and asked if he ever did custom work like this. Turns out he lives not far from us, so we emailed ideas and plans back and forth and then he came out and measured. We are very happy with the result:

A woodworker or cabinet maker could do this for you. Look for ones who have cnc capabilities if you are looking for a really custom shape. They can certainly do it manually, but a cnc would give them nice consistent curves.