Just about any bicycle shop should be able to order a "suitable" replacement from Quality Bicycle Products or one of the other major bicycle parts distributors. By "suitable", if the expected team weight isn't too excessive (350+) and the intended use is leisurely trail riding, an MTB fork will probably be adequate.
I've included a link to the on-line catalog that QBP so you can get a look at the various offerings:
http://bethelcycle.com/site/page.cfm...5&Category=512
You would want to take the damaged for in with you when you order so they can obtain the fork length (hard to know) and rake (probably 50mm) requirments for your replacement fork. It would be good to bring along the make/model/year info and serial number, just in case it turns out to be an odd-ball.
If you find that QBP or the tandem's OEM cannot provide you with a suitable replacement you could have one made by someone like Dennis Bushnell, but it would likely cost more than the tandem is worth, e.g., $100 - $200.