Is not that hard to obtain
The 9800/21 series (or "miko" as I call 'em since I'm a classic 9800 Touhou game lover) used a slot called C-Bus, hence the card's C shape :3
They are inserted like a Nintendo cartridge and they do have screw holes on the back instead of the common PC bracket that we the Americans use
If you go to Y! Auctions and type for example the LAPC-N you will see the card in a C-Bus format
It was like the IBM's MCA and later at the mid/late 90's NEC switched from the C-Bus to standard PCI
Also the 9821Xt is maybe from the transitional era, since it has NEC's C-Bus and PCI, you can notice it thru the dual Voodoo2's (the machine being from 1997 at least I think)
(I know all this since I love that machine together with the short lived X68000 and those two are my favorite Japanese computers and BTW, I would love to get one to play some Touhou)
And as I want to be a nice person with every Vogons member here is how a "miko" card looks like