I know of Pottery Barn, Crate and Barrel, and Ikea, Anywhere else that is reasonably priced? I haven't been able to find them.
Ballard's has several styles.
Seasons Four makes some nice ones.
I have had this place in the back of my mind. When I am ready for new seating I am going to investigate further. Made in USA.
Here is a link that might be useful: Comfortable Couch Company slipcovered sofa
Rowe has several nice slipcovered sofas along with a good selection of fabrics. They are made in th US.