I like nice backings, but am more than willing to run a single seam to make my own wide enough. Sometimes, if it's for a king size quilt, 2 seams. It's very easy to do and cost effective. I look at what Hancock's of Paducah has on sale for $5 or $6/yard. Sometimes, there's a quantity discount on top of that, so I get 10 or 15 yards for as little as $4/yard.
Hancock's also has brand name widebacks. Another place that some find ones they like is
https://backsidefabrics.com . I have not used them, but know people who have.