Hi Laura! I'm not familiar with the 2004 Forest River Flagstaff, I'm assuming it's not a molded lightweight fiberglass rv
At anyrate, we've had several members that have "recreated" the cushions and upholstery in their trailers. I would suggest you start with some cardboard and create a template of the size/shape of the cushions needed. Then either take the templates to an upholsterer or purchase the foam (cut with electric knife) and fabric and DIY.
However, if this trailer is only 3 years old, I'd think the trailer manufacturer could help you out