It also takes two weeks or more to get the stuff. Avoid ordering really heavy things like Kraft paper, etc. The shipping will kill you. We do order from United, never if we are in a hurry to get it though. Usually just hard to find items, hardware, easel backs, etc.
Larson has a lot of the items in their catalog, most of the time it is only stocked in their Chicago location. United is better for those things.
Valley Moulding also has a great selection, however, they are a royal pain to try and do business with, UNLESS you want to pick it up yourself at their warehouse. They just don't seem to want to deliver anything unless they are getting the whole enchilada of your weekly orders.
John