I gave up on stuffing the old cushions. They were terrible. The BIL is an upholsterer and he sewed me up some new ones with new foam. I bought all the materials at an upholstery distributor. Good quality medium firm foam sheets are 1/3-> 1/2 the price of what I can find online. You probably have a similar distributor in a City close to you.
Conventionally, the foam comes in a 24" Wide sheet, 72" Long. Assuming you don't have a foam cutter, an electric carving knife is a good choice. After cutting the pieces needed to dimension, if you need a wider cushion, the foam is glued together with a spray foam adhesive. It's really pretty easy.