Looks harder to find. I can't find any 5v stuff on eBay, just 3.3v
It is tremendously hard to find RAM for these machines, particularly 64 and 128MB sticks. I've been hunting for four 64MB sticks for almost a year now and have found nothing good.
What about adding USB or a pata controller?
For USB the Belkin F5U220 and F5U212 cards are the best, IMO. They work on any PCI Mac with USB enabled in the system software (it is enabled by default on OS 9 and above, I can't vouch for earlier versions).
PATA controllers are another particularly hard-to-find item for old Macs and another thing that I have been searching for for a while to no avail. I'll let you know if/when I find a good card that works with the S900.
That's another mess Does this CD-rom even read cd-rw?
Probably not, no. I've had no luck at all reading CD-RWs or even CD-Rs in my stock S900 CD drive. One of the reasons I want a PATA card is so I can remove the SCSI devices from the S900 entirely. I have a lovely CD-RW drive and a UHD floppy drive waiting in the wings for the day a PATA card shows up.
If you want a more modern storage interface on your S900 then the easiest route to take is flashing a SIL3112 SATA card with the Seritek firmware. It's a tricky process that involves replacing a flash ROM chip on the card but it's well worth it because it means you can boot from modern SATA disks.
I recommend you do a universal install of OS 9.1 on another machine (if you can) and then move the disk over to the S900. It's far, far, easier to get started that way. If you happen to know someone with a G3 Desktop, which has both PATA and SCSI, you can use their machine to handle the SCSI disk from the S900. That said, the original hard drives are ancient, noisy, and painfully slow. I really recommend you try to get yourself a flashed SIL3112.