I purchase wide backing fabric from fabric dot com. Free shipping and they have some great budget wide backs- 108” wide . I usually purchase 3 yards of each I’m getting can easily back 2 quilts plus have some fabric to add to my shelves- or use for binding.

Sometimes panels for the center work well. just add fabric or more blocks to the sides. reversible that way too!

I agree with Iceblossom, just because the cost of the fabric is cheaper doesn't necessarily mean the fabric itself is cheaply made and won't last. I've use sheets, and wide muslin all the time and they haven't fallen apart yet. Of course, we don't want to spend time, a lot of money and energy on a quilt that will fall apart, but I see nothing wrong with getting the fabric on sale or by the bolt to get a better price, so that you're not spending boo coo bucks making quilts that are going to be donated, and for the fact that you don't even know if they'll be appreciated and taken care of. I'm sure that the people who make charity quilts don't make them to fall apart and use ugly fabrics, just because the fabric is cheap, we all make them for someone to love, whom we don't even know, just to let them know that someone cares.

I never buy fabric specifically for "charity quilts" All charity quilts I make are from my stash and my scraps which cost just as much per square inch as the fabric I make for my families wedding gifts. When I buy material for backings, I get good quality quilt shop fabric but do admit to looking for the sale priced ones as the nine yards needed for a large quilt would be a little over $100 dollars at regular prices. Add that to the material need for the front, the cost of thread, and batting. It comes to around $300 dollars. I would love to have no concern over how much I am spending on charity quilts. But I do have to take that into consideration. When I make a charity quilt, the main intend is to provide a home made "blanket" that will be used to keep someone warm and let them know, somebody cared enough to take the time to make it. Most of my charity quilts are scrappy and I do try and make a pleasing pattern, but I do not make "charity quilts" to hang on someone's wall.

Not always but sometimes I will use bed sheets. Twin sheets for the size quilts you are making should do it and with less cost. I also watch for sales at JAF with the bleached Muslin. I will buy a bolt at a time and it last a very long time. I also watch fabric.com because they too offer some good sales.
The other place I look is in my stash where I can sew a panel on the back and then piece together the backing using 45" wide fabrics. My stash gets a lot of charity quilt work.

In general you get a better deal buying 108” wide than with 44” wide fabrics. I like https://www.connectingthreads.com/ backing fabrics selection and they have sales fairly often.

Oh, I was going to suggest sheets too. I just get them at Walmart or Target. Always works for me.

FWIW, when using sheets, get good quality high thread count cotton. Microfiber and the cotton/poly blends as well as those with lower thread count tend to pill quickly and not last as long.

Wow, that last in line fabric is truly gorgeous!! Thanks for showing!!

Flannel sheets work well and I have used regular sheets too. I have heard a lot who prefer not to use sheets but I have found they hold up so well in the wash.