I usually use sheets for backing. I find them at discount stores, thrift stores, garage sales etc. I always have my eyes open for a bargain sheet or fabric. I like to find all cotton but If it's poly/cotton it works too. I like the higher thread count but you can't always have everything. I quilt with my Tin Lizzie, no problems, I do change needles often. I have used 108" wide cotton quilt backing fabric, it's very nice for "special" quilts, but give me a sheet for the price ( used sheet >$3 vs. $30 for the wide quilt fabric).
Mary