I just replaced the cushion foam in my dinette cushions... my original foam had too much "give" and I went with a firmer foam (go figure!). I found an upholstery supply company in Tulsa that stocked several grades of foam, and I went and sat on their foam to decide if it was ok. Maybe you can find a similar company in your metro area where you can try several foams until you settle on one you like.
Memory foam usually is not used for sitting because it compresses too much to offer good support, but a little bit on top might add to your comfort level. If you can get a small piece and put it on your current cushion to try, and if you like it, you could maybe cut your current seat foam down to a lower thickness and add the memory foam. An electric carving knife might work.
To every thing there is a season, and a time to every purpose under the heaven... --Ecclesiastes 3