I don't use the OPT cards in my HDR but I haven't heard of an OPT24. I have only seen OPT-8 cards, which carries 8 channels of I/O in the Optical ADAT format or protocol as I understand it.
Can you describe what it is you are looking for and perhaps I can assist with your search.
I'm looking also. Found this at Thomann, but I can't determine if it's compatible. It was made for the TT24, and since there is compatibility between the D8B and the HDR, I was wondering hmmm...