have a limited life, and upholstery gets worn and out of fashion. Buying
new just makes sense. If the upholstery is in good shape then you can buy new foam, cut to fit (with an electric
knife - check garage sales), and put inside old upholstery. Otherwise, either make new covers yourself or simply have the whole job done by a professional. If money
is tight and you don't need things to be perfect then sewing covers yourself is a real savings.
Whichever way you go you should educate yourself about foam: foam for sitting and foam for sleeping are different from one another - do not switch them as you will not be comfortable. The best cushions are usually made from 2 or 3 layers of different foams (softer on top, firmer on the bottom). Most foam is synthetic, but for a dedicated sleeping cushion pure latex is a luxury to consider. Cushions should be either 5" (general use) or possibly 6" (for sleeping).
Save yourself some money
on the cushions for sleeping by using inexpensive fabric
for the covering - it isn't worth buying
expensive upholstery fabric for that application. OTOH think hard about the saloon
upholstery. It needs to be attractive, comfortable, and durable. It also shouldn't suck the light out of the cabin
- pick a lighter fabric. A popular choice is UltraSuede™, which is handsome, very comfortable, and resists stains well. I bought Alcantara™, which is a similar fabric of high quality, used in expensive auto seating. Generic versions of synthetic suede are available in a wide range of quality/cost.
For the cockpit
, I have been using Bottomsiders™ for many years and find them to be very practical, if a bit expensive. I much prefer them to sewn covers over raw foam.
Good luck with the project