I get my solids mainly from http://www.hancocks-paducah.com - up until recently they were offering 20% off their Kona solids and every so often they offer free international shipping (and you can order up to two batting rolls as well). I also ordered the Kona colour card from them as is a much better way of deciding which colours to order rather than relying on the accuracy of your computer screen. I also use:
If I am after something really specific then I use
http://www.quiltshops.com/ which represents over 200 US quilt shops. Type in 'sale' and you get masses of pages!