If you are any way handy it is easy to make cushions your self without sowing. I redid all the cushions on a 6 M rib I just bought with no experience.
1) Get some 15mm marine ply, cut to the shape you want, include a couple holes in the center to allow air flow. Paint with marine paint.
2) Order the foam dimensions and density you want from
www.foam2home.co.uk , they can cut the shape you want, get the draylon cover and net included. ( Foam should overlap the wood by about 1" on).
3) Order some marine vinyl / fabric to match what is there already. eufabrics.com are based in Birmigham and are easy to deal with.
4) Streach the vinyl out tightly over the foam, (heating with a hair dryer makes it easier to stretch). Staple with 8 mm stainless staples (Screwfix have them). Trim the excess vinyl (loads of examples on YouTube for the method)
5) Add some velcro.