Yes, I've used carpet pad with great success. It is basically just chopped up pieces of recycled foam that have been glued together to form a big sheet. It is nice and firm, so it holds its form really well without bottoming out. You can get a huge roll of it at Home Depot for $40, which will be more than enough for a seat or two.
Use multiple layers of foam to build up the seat, held together with 3M Super 77 spray adhesive which you can get at any Canadian Tire. Don't be tempted to use the cheaper Elmer's spray adhesive right beside the 3M stuff - it doesn't stick nearly as well. Once it is built up enough, use a serrated knife to shape it back down. I recommend your wife/mother/roommate's best bread knife for this purpose.
If you build it up a lot, your seat cover won't fit, but you can get a meter of heavy vinyl from Fabricland for $30. You'll also need a heavy duty stapler to put everything back together.