eBay is the normal way to go here. I ended up replacing a mangled (removed with a pipe wrench, from the size and bite of the markings) adjustable barrel from my Selmer up-end metal clarinet with an A clarinet barrel bought on eBay. Although on a Bb horn, it works just fine.
Metal clarinets are usually found in a badly abused state. This is partially due to the trend away from them by the 1930s, resulting in their being stored in bad conditions for a very long time, and partially due to the fact that many were placed in the case with the barrel installed on the instrument. Just why cases were constructed in this fashion is one of the great mysteries of the clarinet world, but made that way they were, and those who try to play these older instruments often have to pay the price.
(And, this is where a certain individual posts the photograph of his flawless Selmer Bb and A full Boehm clarinets in glorious silver-plate. Sometime the internet is a conduit for envy...)