There aren't any OEM ADAT A/D's, every manufacturer makes their own converter boards. Well, maybe some chinese companies might have something but maybe not worth much. Some manufacturers like RME used to sell io expansions which have ADAT I/O:
http://www.rme-audio.de/old/english/aeb/aeb48i.htm
Also Focusrite has some expansion card for their mic pre's.
It wouldn't be hard to design an ADAT A/D card using the new AKM 8-channel A/D rkn80 is using, though there should be a way to link multiple of cards using word clock or sharing the same master clock.