The foam can be cut with an old electric knife like they used in the 70's to cut the turkey. (only time they came out in my house growing up) You can find them for 2 or 3 dollars in most thrift shops. If they are not really visible then just get a foam mattress from some place like overstock.com. they sell one made for "rvs" that looks good and cut it to match the existing cushions. It comes with 2 layers of foam for around $300. I would save the existing covers in case you want to have new ones made.
Why use sunbrella in a v-birth? Does not seem comfortable. Any synthetic upholstery fabric should work. Should not get a lot of UV down there.