JoAnn and Hancock Fabrics both carry upholstery material. Lots of info on the internet on how to do upholstery. Bad thing about upholstery fabric is that it needs to be dry cleaned. Denim and painters canvas (drop cloth) can be machine washed and is usually cheaper. If you use Denim or Canvas, wash & dry it before cutting and sewing so that it will shrink. Bed sheets make good slip covers... cheap and machine washable.
Found a link that I was looking for. I came across this a few years back and had printed it off (I keep it with my "bus stuff"). It shows a simple seat that I believe you can expand and use as a bench seat. It is removable which means when it needs to, it can easily be recovered.The T-nuts in the plywood seat platform means that you can attach the seat so it doesn't slide off the chair/bench. I think you can use the same design to make padded seat back cushions. I haven't built a seat like this... yet. I plan on using the same basic principle to build a new seat & back in my old, decrepit, pre-1900's rocking chair that I have to modify to work in the bus.