I bought new Captains chairs from DiscountVan and RV in California. They will custom design them with a foam that suits your needs. After my back fusion, I can still drive all day with the new seats.
There are also several places that handle air ride semi truck seats that may work for your needs.
I also find that having a seat with an adjustable seat base, where you actually sit, that allows you to vary the angle of the seat in relation to the back, also helps in providing some comfort. This can be used to relieve some pressure on the back of your legs by your knees.
Basically, besides having the correct foam, I like a seat that is adjustable in all directions to vary my driving position in order to minimize staying in one position too long.