I shop a LOT online and I have bought quite a few kits. I love:
shabbyfabrics.com - there quilt kit was the best labelled so far, great customer service
fatquartershop.com - been very impressed with service and quilt kit packaging
I also love
http://www.grandmasatticquilting.com/ - I love the little extra they put in with your order.
Ebay has been great - had no problems.
I can't think of any others at the moment. They are the ones I frequent most.