I replaced both of the seating halves on our dinette earlier this year as it was bottoming out when I sat on it. I took it to an upholstery supply shop in the area where I chose the better/denser of the 2 types they carried. Huge difference in comfort, should have done it years ago. The guy at the shop took the measurements from the one I brought in and cut it on the spot for nothing. It was left to me to stuff it back in. I also covere it with 1/2" batting to keep the foam from sticking to the fabric. Cost me about $50 for each 4' x 2' peice of 4" foam. The fella cutting it did not want just the foam brought in but the complete cushion.
When I looked into just ordering foam off the internet shipping made it impractical, much cheaper finding a place around you. I was told by another shopper that Joanne's Fabrics carry it may be available at most fabric places. If you go this route read up on the different densities on the internet so you know what you want.
