Quilted Twins and Marshall's Drygoods. Have purchased from both and very pleased with the quality, choices and prices.

i usually start at https://marshalldrygoods.com/

i had never looked at https://www.quiltedtwins.com/, so took a peek.
they appear to have all their backing fabrics on sale at a deep discount.
i can't find any information about how long the sale lasts.

PatriceJ the quilted twins sale price are their regular prices all year long.

The sales at Joann Fabrics usually don't include the quilt backs so the 50-60% off coupons work on them! Yes, you can do it online.

Not a huge selection but you've got to love the sales price.

Several vendors. Like Onebyone, I tend to buy when I find it on sale. Or if I am in the Fingerlakes area as the Mennonite shops have it very inexpensive. Here are some direct links for you:
https://marshalldrygoods.com/shop/home/108/widths/
https://www.quiltedtwins.com/backing-fabric/
http://www.hancocks-paducah.com/SALE...CE&sort1desc=F
https://www.thousandsofbolts.com/ind...isplaycount=80

Also don't overlook extra wide Muslin. This is usually very economically priced and I have used it for backings quite often with very satisfactory results. Marshalls carries it.

I prefer a solid or a fabric that reads solid for the backing so the quilting will really show but if you are not super confident in your quilting, choose a backing that is a busy print so it hides any little imperfections.

Marshall's has Dream Cotton muslin color in wide backing. I keep a bolt of that all time. Best value for the quality anywhere.

I use a very good muslin that is about 108" wide. Quite a few places sell it. Just google. I think mine is Kona Premium which I also use for background.

Wow, Quiltedtwins have 118" wide muslin at $8.49/yd! And its pre-shrunk! Will have to try them next time! Thanks Patricej.

Amazon, which is usually means it comes from Fabric.com or Hancocks Paducah

Christian Lane Quilters is my favorite site for wide backings. http://www.christianlanequilters.com/home.html and Thousands of Bolts has wide backings too. https://www.thousandsofbolts.com/index.cfm