I have purchased backings from here. Nice material!

KeepsakeQuilting.com has several extra wide backing fabrics. I've also purchased some at Hobby Lobby. The woman at Hobby Lobby said they're going to begin stocking a wider variety of 108" wide cottons.

Fabric.com or Hancocks of Paducah if I shop online. In Illinois I love Boline’s in Bloomington..lots of choices for about $8 a yard.

I purchase 98% of my wide backing fabrics from fabric.com
they have a large selection, good prices and a free shipping price point I can always meet. I’ve been shopping with them for 20 years , their customer service has always been good, very few problems over the years (3-4 in 20 years) problems were always solved/ fixed/ handled quickly and cheerfully.

I usually get mine from Hobby Lobby. I can see actual color and feel it.

I get most from Amazon, which is usually Fabric.com or Hancock’s of Paducah.

I get all of my 108 inch from Marshalls the quality is good and I love the choices that are available and at $7.99 a yard you can't beat it.

I get mine from my one of the stores in Amish country, Shipshewanna, Indiana. No help for you but for me a hour drive 3 or 4 times a year is worth having a marvelous large selection of wide backing. One of my LQS also has a moderate sized selection that is growing as more of us request this fabric. You have so many web suggestions to investigate. Have fun making choices.

Alternatively, you can get a good quality cotton bedsheet in a suitable size from the usual sheet sources (department stores, websites etc.) - I wanted plain fabric for my backing, so that's what I did!

thanks everyone for your input. I did not realize there were so many choices