I picked these up from eBay. The Prose 2000 was pretty easy to get going and I have it talking over serial connection. There's still a few positions on one of the dip switches that I don't know what they do, and if there's anything over the serial other than plain ordinary text and punctuation (i.e. special commands to alter speech parameters), then I'm unaware of them.
The iSBC-576 is another matter entirely. From the product sheets on Mark's website I can tell that it's a speech recnognition (and synthesis? I'm not sure) card that would have come with an external box, microphone, and some ISIS software. All I have is the board, which would make this one an uphill battle.