Lucky me I don't need to go online for extra wide backing. I am about an hour from Shipshawana, Indiana and a couple times a year I meander down and by wide backings from there. I also have a shop that has a good selection if I need something in a hurry. I also have been known to purchase a bolt of JoAnn's highest quality bleached muslin from time to time. I often use white so it works for me.
I will be watching your thread for online suggestions too.