I recovered all our cushions along with new foam too.
Ditto sailrite.com; it's the best source for everything including advice.
I used upholstery fabric from the local fabric store. If the cushions are for outside, use the solid foam colored ones and skip the fabric. For interior, I think that sunbrella is scratchy.
One bit of advice - use an open mesh fabric for the cushion bottoms to let them breathe.
More advice - In winter, take the cushions off the boat and store in the attic. They won't smell like a mildew convention.
Sabre 38 "Victoria"