I've purchased from both Interstate and Port Plastics. I've found the products from each to be nearly identical, so I will usually call both for price quotes, asking specifically about the shipping costs, and if there are any handling or cutting charges applied. In most instances, Port Plastics did not charge for handling/cutting, and their shipping was SLIGHTLY cheaper.
The material that I normally order is kydex 100, and most often purchase two full sheets at a time, which they cut into quarters for shipping.