using your original seat pan....
you can DIY with hardware store foam, spray glue, and a carving knife. shape your own, and have it custom upholstered..
You can have an upholsterer do all of the above...
you can buy a new cover on the internet and add it over the original foam
you can buy a whole foam pad and cover kit
or, you can buy a whole new seat(used, repro, oem)