1994 Santana Sovereign tandem, 1999 Trek 5000, 1985 Specialized Allez SE
I've researched this a bit (being the owner of an older Santana), and I have not found a lot of options. Your best bet may be to find an old broken tandem with a good fork. If you are looking for a new part, your choices are limited:
Santana sells a 1" threadless tandem fork for ~$300. That may be the only new tandem rated 1" fork you can find (I'd love to hear otherwise).
Surly makes the Cross Check fork in a 1" threadless. You can find it for ~$70. While sturdy, it is not tandem rated.
http://www.rivbike.com has a 1" threadless Carbonomas Fork, but again it's a road fork, not tandem rated (I asked).
You can find various "generic" 1" road forks, but I'm not sure I'd put one of those on a tandem.