I'm pretty sure that with the exception of a few "demonstration events" such as the one held at Trexlertown, PA each year, track tandem racing (e.g., Tandem Sprint) has all but disappeared with one exception: Paralympic track tandem racing for disabled (usually sight or hearing impaired).

The demise of tandem track racing started when it was dropped as an Olympic event after the 1972 Olympics at Munich, Germany. I'm not exactly sure when it happened (1995?), but UCI and USA Cycling also no longer sanction non-disabled track tandem events / the tandem sprint as they have been deemed "too dangerous".

However, paralympic tandem track racing is alive and well...

http://www.paralympic.org/release/Su...les/index.html

http://www.usaba.org/

http://www.ibsa.es/eng/deportes/tand...esentacion.htm

As for finding a track tandem, there are a few out there and your best bet is checking with the folks at USABA and/or the velodromes around the country to see what's available. You can also "Google" on the phrase "track tandem" for sale and see what comes up on both the Web and Groups.