If you found one as a kit... I would get that immediately. You won't find this at Guitar Center that's for sure. They were in short supply even when the machine was new, and the best option was to buy a machine that already had it..... There have been a couple of people that have engineered their own, but those are few and far between. Again, if you found one snatch it up because it may be a while before you see another without buying an entire unit that has one...
You could always reach out to Clark at Rubber Chicken Software.. An Ensoniq genius
Here's one: you'd have to DYI, AND it's more than what you've found one for
SCSI INTERFACE BOARD 4 ENSONIQ ASR-10 ASR10 KEYBOARD! - eBay (item 390155240012 end time Mar-12-10 04:15:31 PST)