dealers and manufacturers will be able to provide replacements for about 5 years. after that, ebay, craigslist, or the upholstery shop or self upholstery as indicated.
I use a local upholstery shop. They specialize in boats and classic cars. I had my jet ski seat redone, sundeck redone and a cushion redone by them. They don't charge too much and use marine vinyl. The sundeck has a swirl pattern to it and they matched it perfectly to factory. Just look for the shop that has a bunch of boats. When I had mine done I took them off the boat before storage and said you have until May. They are usually cheaper that way because it just fills in time they don't have anything to do.
Depends on how much you want to spend, how old the boat is, and if the originals were customized to the boat. I use a combination of building my own then taking them to ol' Gordon if they have any sewing. If just a square / rectangle wrap I do myself. Materials to be had on line.
On the Carver I did use Veada for some really nice helm seat - and they were very helpful in getting me the right ones; not a lot of room and wanted to be able to spin around.