Sunbrella or another brand of outdoor performance fabric should be used for all seating upholstry if you want them to last more than a season or two. Sunbrella is as close to bifl as fabric gets, imo. I have white Sunbrella cushions that have lasted for years and still look almost new.

Overstock and Wayfair both sell Sunbrella outdoor cushions and pillows if you cannot find them in your area.