Just get any 9.5mm hard disk from eBay (i.e. just about all hard disks).
Possible options are the DJSA-205, 210 or 220 drives (5, 10 or 20Gb respectively).
Also a drive starting IC25N would be fine.
Any drive is fine so long as it is 9.5mm. You should probably go IBM or HGST as a brand though as some IBMs (namely the older ones) are difficult with other brands (the ones I listed are IBM/HGST). Make sure when you install things on the drive you set it active to make the IBM read and boot from it.