Precision Tandems:
http://www.precisiontandems.com/catframepart.htm
Gear to Go Tandems:
http://gtgtandems.com/partsnew.html
http://www.crank-2.com/store/search.asp
FWIW, I've only actually dealt with Precision Tandems. Not sure about close out stuff, though.
__________________
Jeff
Learn from other people's mistakes. You won't live long enough to make them all yourself.
2004 Co-Motion Speedster
2010 (Specialized) Carmel comfort (my neighborhood bike)
2008 Raleigh comfort (wife's neighborhood bike)