Check out foamfactory.com. They will make you a mattress of any dimension and they have 4-5 levels of firmness. We've used them to replace all the cushions in our 1996 Bigfoot
1500 as well as to make a custom mattress for our 30 foot sailboat.
In the case of our Bigfoot
, we purchased the foam cushions from them with dacron wrapping and had a local upholsterer cover the cushions using Sunbrella fabric we also purchased online. Can't recall the exact Sunbrella site but recall there are several that sell it online - might have been in Florida. Overall project was half the price of the quote from the upholsterer and several local RV cushion shops. She ended up charging us $60 per cushion for her labor to cover the cushions
For our boat mattress, foam factory added the dacron wrap and the covered the foam cushions with blue and white mattress ticking material. The total cost was a about one third of the cheapest cost from other marine mattress suppliers
They ship the cushions with all of the air sucked out and then wrapped in heavy duty plastic so it essentially a brick. Once you slice the plastic it assumes its intended shape an about 24 hours. Good Luck