It's extremely unlikely you'll be able to source this now, unless someone is breaking a 381i for parts.
I managed to get what was probably the last digital input board supplied by Wadia for my 830 back in 2011 , but it took me about 6 months of chasing various Wadia dealers here and in the US. None of them including Wadia themselves had a clue about that stuff even back then as the company had already been bought over.
I eventually got the input board from the US dealer (now closed) but it didn't come with the firmware chip required to make it work and they eventually just stopped replying to my messages about it. I managed to get a new firmware chip from the UK dealer (also now closed) but it hadn't been programmed correctly and didn't work! Luckily I know about that stuff and was able to work out what they'd done wrong when writing the chip and program another one myself that worked OK, but first I had to buy a specialist chip programmer to do it as they were using a quite unusual type of chip for the firmware.