E-Bay for a used bellows, and probably Camera Bellows or Western Bellows for a brand new bellows.
You should be able to get bellows on ebay in excellent condition for about 20 to 30 percent of what they will run you new. (My Sinar bag bellows was under $80 shipped, and in unused, perfect condition.) I don't know if Linhof bellows are all made of leather or not, but all the ones I have seen are, and tend to get a bit ragged with use. If so, an older used bellows might be more dodgy, and I would ask for nice macro photos and a light-tight test. You might just want to take this opportunity to "upgrade" to a synthetic bellows instead, and just go ahead and order from a custom shop like those mentioned.