I know you can swap bottoms from passenger side to drivers side... several threads on here and success stories.
I'd rather find a replacement foam but I've only found one thread on the entire internet mentioning this, and the guy didn't disclose where he found it (it looked new, though - and I think he said it was a gift from his dad or something). I tried to find seat foam replacement and failed. I'm sure a custom upholstery shop could do it but I don't envision that being cheap.
There's also a youtube video floating around of a guy restoring seat foam with a steam cleaner. I guess it puffs up a little with the moisture and then dries, retaining a more restored shape. I don't have a steam cleaner nor much faith in this method, mine feels entirely destroyed from previous owner's butt scooting across for 240,000 miles and then my butt scooting across for the last 10k miles with no running boards and increased ground clearance.
In for suggestions...