Derek,
I don't know of any board that will provide 100's of serial ports. Digiboard would be where I would look first but it seems like an awful high number - just physically getting, say, 200 serial cables connected into the back of PC sounds like "mission impossible".

I thought we had a lot of serial connnections with 10 or so!!!

Anyway good luck with the search - let us all know what you find.
Sam