Where can I purchase a trailer bike for two kids at?
(Photo from Greater Ohio Area Tandem Society Web Site)
I hunted down a few for another gentleman a couple years ago. As you may have already discovered, they are out of production but there are a few of these still in circulation, mostly 2nd hand. They were produced by NORCO and while they work, they are marginal at best in terms of how they handle and affect the handling of the tow-bike.
Your best bet is to subscribe to Tandem@Hobbes -- a much older tandem listserv with a very large audience -- and post a note requesting leads. List info and links to the subscription instructions are here: http://www.thetandemlink.com/hobbes.html
You could also post a "wanted to buy" ad to http://www.tandemmag.com/classified but I don't believe it will yield as many leads as a post Hobbes, which is where I had some success during my previous search.
Adams tandem trail-a-bike is no longer being produced. As Mark sez: look for a used one.
Another suggestion: buy 2 tandems and set each one up for a kid-stoker . .
I know a store here in my town that has one on display. We actually bought one from them last year and have enjoyed it the times we have used it. His "Sale" price is $299. Hope this helps.
PS - Handling is somewhat 'trickier' than say a single trailer bike (as we had one when we had ONE [child])