Oem digital devices

Does anyone know of an oem 8 channel a/d  or a d/a that one could add onto an existing design?
I know they exist I just don't know where to look.

I believe Mini DSP might have what you need:

https://www.minidsp.com/products/usb-audio-interface/u-dac8
https://www.minidsp.com/products/usb-audio-interface/u-dio8

I don't know if this is what you mean by "OEM", though  :

There's also Böhmer:

http://bohmeraudio.com/download/Bohmer_Audio_Hardware_Specification_v2.pdf

A big question is what's required on the digital side? USB, spi, ADAT...

adat or aes.

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.