I got my seat covers and new foam from Corvette America, the Not Quite Perfect original style leather were reasonbly priced and turned out looking great.
Some advice:
Always replace your seat foam when you do the covers, the new ones won't fit right over tired foam, and you will definitely regret not doing both at the same time.
Do it yourself, but be prepared for some hard aggravating work, it will save you the money you'll be spending on the foam.
Take the time to clean up the entire seat frame, remove and rust and repaint it.
You don't have the seat apart but once every 10-20 years so do it right the first time. There are quite a few threads regarding the replacement, more so in the C3 forum.