First place to check is your owners manual for the furniture set, to call or email the manufacturer about ordering replacements. Second option - if it's just the cushion covers that need replacing, there are a number of shops on Etsy that will sew custom covers. If it's the cushion/filling that needs to be replaced, do a google search for upholstery foam cushions. There are a number of foam manufacturers that will cut custom size replacement cushions. For that matter, you may have a shop locally that will do it. Prices will vary depending on size and quality/density of the foam. Be warned: the cushions are frequently the most expensive segment of outdoor furniture, so they will be surprisingly expensive to replace. More on reddit.com

Sewing replacement cushion covers isn't terribly challenging, so you may want to look for someone locally who does custom sewing jobs. There are also many people on Etsy who make and sell custom patio cushion covers. You provide the dimensions, choose the color/pattern from their fabric selection, and they'll make them for you. More on reddit.com

I don't own any of the conversation sets sold by Costco, but I have four of the Nardi commercial patio chaise lounges from them, and they're AWESOME. Buying them from Costco saved me about $400. More on reddit.com

My advice would be to think about your plan for winter storage. If leaving them outside all year then pull the cushions inside so rodents don't nest and then get specific furniture covers for outdoors from Amazon. More on reddit.com