There are a lot of great threads here on upholstery, etc. I tried using the search and entered upholstery and cushions. It came up with a bunch of threads some of which are instructional, some just praising their new stuff. But that would be a start to getting more info on what you want to do.
If you are a person who already sews, has a machine that can handle a heavier fabric, then go for it. Usually when you go into a fabric store that has upholstery stuff they either offer classes in how to make seat cushions etc, or they have someone working their knowledgable enough to give you a few pointers. Be sure to use fabric that is made for upholstery. You want it to withstand the wear and tear of camping life and look good at the same time.
As far as where to find supplies...you can find a good deal of fabrics on-line. The list on Vintage Airstream was helpful to us in finding the fabric we wanted. VintageAirstream - Suppliers
There are also links to online foam companies there. But we live in a place with limited resources, we had to order our fabric online. In Tampa, I'm sure you can find stores that would offer you the foam and fabric that you need for much cheaper. The shipping on large quanities of foam can out weigh the lower cost of the foam found online. So buy local whenever you can.