The pricing posted by Alice seems about right based on my experience with prior boat. The earlier post pf $1,000 for one seat seems way out of line. But the question about how to go about finding someone to do it - I would go to a local marina and ask who they use to do it for their customers or when they take a trade in that needs some work. You want to be sure they use marine grade material and have experience doing it. As mentioned, check out to see if you need new foam. I did not do it on previous boat and then the seat back did not quite reach the seat (as designed on that boat) and it left a gap.
Unless you do not mind paying shipping there is no need to locate someone out of state. Just inquire at a marina.