Uhuh, uhuh.
Well, Fostex was supposed to supply the firmware, usually by Email or so I've heard, but if they don't do that any more, then I guess you're SOL. You can read the FW update procedure, but as I recall, it involved putting the FW on a compatible cartridge drive on your puter, such as a ZIP or SYQUEST, and then using the compatible SCSI-external cartridge drive as a source drive, to pull the firmware from. It's a bit complicated, as I remember, but nothing approaching rocket science.
The FD8 Service Manual has a section where it describes how to go into Service Mode, which is where you check your current version of firmware. It may be that you already have the latest version, but unless you read the manual and check yourself, you won't know how to proceed.
I believe there's a list of compatible HD's for the FD8 in the Service Manual, but it's been a while, so don't crucify me if I'm wrong. I know I've seen HD info somewhere, and I believe it was the Service Manual appendix info, to the best of my recollection. Even so, any list would likely be outdated, so again, you're on your own.
I opened up my FD8 a while back, and to the best of my recollection, it was a Toshiba 4.3Gb drive. I just searched Toshiba 2.5" drives, up to 30 gig, but I never sourced one,... for other than technical reasons.
It basically boils down to, you gotta read the manual before proceeding.