Not sure how close you are to a Nebraska Furniture Mart, but you can have them make any size you want in their house brand of mattress for a $50 up charge. I believe the name is Omaha Bedding.
Only problem we had was we didn't think about the overhead cabinet above the bed. We chose a pillow top mattress. It is much thicker than the original one. We have to be careful getting in and out of bed or you get a painful reminder that there is a wood cabinet above you!!!! Only took once for me!!! Good luck, John