I haven't found any covers per se. Best I've seen are complete pieces on eBay. Your best bet may be to find a local upholstery shop. If you're replacing everything, it will be a few grand.
I spent last winter carefully removing the upholstery from the 17 cushions on my challenger 210. Thousands of staples removed one by one. I did one or two each day after work.
I gave the pieces to a seamstress at a local furniture shop. She used the existing pieces to pattern and sew new pieces with vinyl I gave her. I then did all the work to put the new upholstery back on.
I had never done upholstery work before, but it turned out great. I spent about $750 on high quality vinyl. She sewed them for $50 per seat. So, it cost me about $1500 and a bunch of time over the winter.