http://www.tascam.com/Products/IF-TAD.html You should be able to find WR-TDIF (TASCAM) cards very inexpensively.
www.nivie.com Several third party manufactues such as TASCAM, Fostex, M-Audio and SPIRIT (Soundcraft) make TDIF to ADAT adapters.
IZ Corp/OTARI and RME make a 24 channel format converters but they're expensive.
good luck,