If I have enough I use yardage I have on hand. Have bought the 108" wide. If I can find a good quality Muslin I use that. I don't use sheets.

Try Fat Backs, or Back Side Fabrics. Back side fabrics will send you free swatches About 2 1/2 by 3" in a few different fabrics so you can see if they are the correct color and how they feel. You order them just like you are ordering the full amount. I've ordered from them several times with good results. Quilted Memories LLC also has some lovely wide backing.

As long as a quilt will not be hand quilted, I have bought white sheets and dyed to match, or bought nice used sheets from St, Vinnie's, to back quilts with. I buy king size sheets so I can use them on any size quilt. Just be sure to completely unfold them to check for stains or holes before purchasing them.

I used unbleached muslim for all old patterns and bleached muslim for new patterns. or new sheets. for king and queen size. washed first. I hand quilt mostly or SITD. I am starting to do QAYG. so will use new sheets for that. CMARAS1234

I like backsidefabrics too. They have a variety of choices: 106" 108" and 110" backing sizes at great prices. Lots of choices, & good quality. I buy this backing fabric to use on the front (some for under $10/yd):
http://backsidefabrics.com/value-col...uilt-backings/
I don't work there...not affiliated or anything...just like em.

I also like Thousandsofbolts.com She has good prices and they are all name brand fabrics. I have had great service and the fabrics are good. I love the name of her shop: Thousands of bolts, One NUT!

I have bought wide backing from Hobby Lobby and from Hancock's,( the chain store Hancock's) using a coupon and that makes it very reasonable. It is good fabric.

The prices at Whittle's are outstanding. Also check out the sellers on FB, there is a variety.