For backings, buy sheets on sale. I've found that 100% cotton or 60/40 cotton/poly blends work well for that, as do flannel sheets. I usually get flannel sheets from Sears or Penney's late January and February because they are on sale. But look them over carefully so you can get the best quality flannel you can afford.
If you have a Good Will near by, check for fabric there. I make a trip to the local GW to check for fabric at least twice a mont. It's hit or miss, but I have been lucky enough to get nice fabrics for a very low price. Look over the sheets there, too, as some are new or like new. In fact, I sometimes find the top sheet of a set of sheets that has never been used, while the fitted sheet shows wear and tear from use. Apparently, a lot of people don't use the top sheets.
Walmart has some good fabrics, but like any quilt fabric store, some of it isn't so good. Since you're an experienced quilter, you will know the difference.
Hobby Lobby - most fabric is nice and the prices are reasonable. JoAnn's - used to be reasonably priced, but last time I was in a JoAnn's, the fabric was pricey.
Online stores: Try Connecting Threads, Marshall's Dry Goods, Thousands of Bolts. I'm sure others will have their favorites.
And, of course, shop the sales both at LQS and online.