The Original Mattress Factory - will make them for you custom. Not too bad on the price either. Don't let them sell you 12 inch thick foam though - too high, too heavy. 7 inch is fine, or they'll also do 9 inch.
If you can't find one in your area, do the yellow pages thing for mattresses-custom. Also many auto upholstery places will do this kind of work.
Today is a gift, that's why they call it the present.