I purchase Walmart's Heavyweight Unbleached Muslin in 5-yard lengths. I use it for ironing board covers and pressing mats, as well as quilt backings, hand embroidery, and bag linings.

I like Cranston fabrics, which I've found at Wal-Mart....another store that would probably be condemned for not carrying "quilt shop quality" fabric. and I like Kona, which I get at Hobby Lobby. I've also bought quality fabric at JoAnn's. Quality fabric is quality fabric, wherever you find it; experienced sewers/fabric nuts can tell by the feel.

Another store to add for your muslin search is Quilted Twins. I have not purchased the muslin but have a few of their bundles & they are very nice fabrics. I had an email a few days ago that they have 118" wide muslin at a very good price.

I know I have a bolt of bleached and unbleached muslin..........not from JoAnne's though and yes I have used it for background from time to time. I also use it when making strip blocks for the backing.