Also, Lowe's and Bed, Bath & Beyond. I've noticed that both have displays of interchangeable covers -- might be the same manufacturer. I don't know the size, tho. And you might also consider making your own. I am not all that skilled, but I did make some envelope-fold ones with Velcro closures in the back. Super simple, and you have a better chance of getting the fabric pattern and color you want.

Agree with woodteam and Penelope...I go to Home Goods or Tuesday Morning and root around the clearance section. I've found great down/feather pillow "stuffers" there. Plus the fabric that the pillow had on it can sometimes be used for something else or I donate to Goodwill.

The way it works is that the cover you're buying will fit a 20" x 20" insert. So they give you the size of the insert, not the cover, per se.

Love that William Morris fabric! I like the navy. I think it brings in such a richness of colors and nice contract with the sofa, as well as a great backdrop for the WM print. An idea of the grouping. I know you have the light blue pillow already; I just wanted to see a couple of others for fun. I’m loving the rust one. ETA: Cream. This does look really pretty, too, and more casual than the navy. I like it a lot. It ties in well with the rug. Cream with rust: Flax. I’m not loving this one. It’s blah to me. So after doing mock-ups, my vote is a tie between navy and cream. :)