I don't use K-cups but I do use pods. I usually by mine online at the following site. They have great variety of single serve coffee, Pods and K-cups.
http://www.coffeewhiz.com/SearchResult.aspx?CategoryID=76
Also an excellent resource for Single Serve coffee, they have great reviews on the coffee brands as well as the coffee makers.
http://www.singleservecoffeeforums.com/