Try:

http://www.thetandemlink.com/LearningCenter.html

Good used CoMotion or Santana can be had for $1500-$2,000 if your look carefully. I just sold a nice clean 2006 CoMotion Speedster with some upgrades for $2,400. This bike was in great shape and not a budget tandem when new so you could go cheaper.

See

http://www.tandemmag.com/classified/

Good luck - Wayne