My opinion, Walmart has pretty WOW fabric (White on White) and COC (Cream on Cream) which I personally buy there by the dozens (on the background fabrics) but if you cannot find these background fabrics, then Joann's will have it (second option for me if I cannot find a nice background fabric to work with).
Right now 5bucksayard is having a tremendous sale; here's the link for the homepage and if you sign up for their email alerts, the site offers incentives/promotional sales/clearance/etc...
Here's the link:
http://www.5bucksayard.com/