I don't think I could sum it up better than Chris. Eurorail Hobbies has all of the Faller and Vollmer kits. Some of their items are usually cheaper than Walthers. Their site is easy to navigate so that you can find what you're looking for very quickly. Since Eurorail is in Canada, I don't know how customs are handled for U.S. customers.
If you're trying to hunt for a possible bargin, than Ebay is for you. Not only you'll find the current Faller models but some of the discontinued stuff is listed too. Becareful that others will big against you since Ebay is an auction site. Especially for discontinued models such as the Tea Cups and Mammoth Hole kits. The majority of sellers are from Germany and some do understand English. Many will only ship withing Germany since the shipping is expensive. If they are willing to ship to the U.S. or Canada, ask if they can ship it via surface mail. It's much cheaper than shipping by air but it takes about 6 to 8 weeks. You'll also have to mail them cash in either Euros or USD (ask first) since they don't use PayPal. If they'll take a Euro bank draft then go for it, but do not send them a money order. There's one seller who lists from Canada and his opening bids are even cheaper than Eurorail Hobbies.
Finally, if you're looking for Cummons quality models at a much lower price, there's Renauld's Midway Miniatures. He mainly deals with American classic rides. All of his models are pre-assembled, non-motorized and you can request what paint scheme you want.