Several vendors. Like Onebyone, I tend to buy when I find it on sale. Or if I am in the Fingerlakes area as the Mennonite shops have it very inexpensive. Here are some direct links for you:
https://marshalldrygoods.com/shop/home/108/widths/
https://www.quiltedtwins.com/backing-fabric/
http://www.hancocks-paducah.com/SALE...CE&sort1desc=F
https://www.thousandsofbolts.com/ind...isplaycount=80
Also don't overlook extra wide Muslin. This is usually very economically priced and I have used it for backings quite often with very satisfactory results. Marshalls carries it.
I prefer a solid or a fabric that reads solid for the backing so the quilting will really show but if you are not super confident in your quilting, choose a backing that is a busy print so it hides any little imperfections.