I did the seats in my boat last summer. It wasn't an easy job, but the local shops wanted more than $800 to do it.
You will need "4-way stretch" vinyl. don't even waste your time with regular marine vinyl.
There is a member on this board that makes replacement vinyl for your seats. he is reasonable, and fast... but you have to do the labor for the installation.
Do a search, and you will find him. I don't remember his name... but he's down in Florida.