Have you tried to simply search Slipcovered Sofas in the HOME FORUM? A number of posts come up that name the various companies that manufacture and sell them, and posters opinions about them?

I did try the search before posting but the vast majority of the discussion was pre-2010. Plenty can (and has) changed in 3+ years. An opinion from 2008 is of no help whatsoever.

Here is a list of manufacturers as a starting point. I have also included the link to photos of slipcovered sofas in Houzz because you can view the picture, and ask questions. They will often identify the manufacturer of the furniture. You might also want to post on the Home Decorating forum. They might be a bit more chatty about this.

Mitchell Gold & Bob Williams http://www.mgbwhome.com/slipcovered-C13.aspx
MGBW also private labels some of the sofas sold by the retailes listed below. They are available nationally.
Rowe
Pottery Barn
Rooms To Go (Cindy Crawford Collection)
Arhaus
Ballard Designs
LL Bean
JC Penney
Restoration Hardware
Room & Board
Club Furniture
Nantuckit
IKEA
Crate & Barrel
Home Decorators Collection
Cottage Home
Lee Industries
Our Boat House

Here is a link that might be useful: [Slipcovered Sofas on Houzz[(https://www.houzz.com/photos/white-slipcovered-sofa-phbr0lbl-bl~l_42878)

Many of the aforementioned are retailers and not manufacturers. I like Four Seasons and Lee Industries

Know this is an older post, but thought this could be helpful to someone. Just purchased a slipcovered sofa from Club Furniture. I got the Camden style in the Bella microfiber velvet. It seems quite well made, happy with the slipcover stitching, etc. I also added the feather-wrapped seat upgrade. Love the size, comfort, was delivered right on time (6 weeks) from order to receipt. Loved the ease of ordering online. People on the phone are quite pleasant and helpful. Thumbs up! (I actually ordered a leather sofa from them about 10 years ago, with same good result). Thinking of ordering another slipcover for the summer.