The Linhof 617 and 612 are fairly rare cameras on the used market. I see maybe two or three of each come up for sale used each year, usually on EBAY. Sometimes these have been bought for projects, then sold after the project ended. The Fuji 617 is slightly more common, though not much lower in price.
In the United States, B&H Photo in New York City is about the lowest pricing from a reasonable dealer. On the West Coast, you can find seveeral Calumet Photo places, with good reputation, though probably not as good pricing. The Linhof 617 is something that would not be likely to be in stock, except maybe at B&H Photo. I know Nelson's Photo Supply in San Diego could order one, with pricing close to B&H Photo, though there would likely be California Sales Tax on top of the price.
Gordon Moat Photography