I see stuff like this pop up at my local Goodwill this time of year. People are looking to get rid of it rather than store it another season. Also, the big box stores seem to have a lot of mark downs at the end of season. In my area, we have Lowe's, HD, and Menard's. Also Big Lots and Ollie's seem to carry a lot of outdoor furniture.
Oh and if you have a Habitat for Humanity Restore nearby, check it out. The one near me always has stuff like this!