Generally speaking, 50-pin scsi drives larger than about 4GB are rare (based unscientifically on the fact that I don't own one). I once came across a large stack of 4gig drives in a surplussed server. Bought 10 at a buck apiece and upgraded my old 68k macs. Wish I had bought more (there were maybe 50 of them, total). When I got around to returning to the store, they'd all been sold.

Good luck tracking down these larger drives. They're out there, but getting more scarce day by day.