I have nothing to gain suggesting this company, other than they were less than 1/2 the price of my local foam shops when I went to replace the foam in our sofa. They sell a wide variety of foam and cut to size.
Foam is kind of made in 2 different ways, one way is how firm the foam is, how much weight it takes to compress it. The other more important way(IMO) is the weight of the foam, heavier the better. You can have a cheap light foam that supports a lot of weight, but breaks down quickly(think cheap sofa). Then you can have a heavier weight foam that is either soft or hard, depending on your weight. For our sofa I chose the heaviest foam I could get, in a medium to higher compression weight, it's great! For something like a car seat I'd suggest heavier foam and purchase the compression rate that best suits your weight.
I see you also have an account on the Bolt forum, if you deem appropriate you might want to post a link to this foam company. Again there might be others but I checked quite a few and these guys were the best priced for good to great quality foam, and they totally cut to size for N/C, at least they did for my sofa cushions.https://www.buyfoam.com/https://www.buyfoam.com/OurFoam.aspx