edit: Now I have got an Adtron SDDS SCSI PC card drive (3.5 inch form factor, 5 V supply voltage). Single and dual PC card drives of this make are still available, but manufacturer lists item as "end of life". The device is fitted into a nicely yellowed external SCSI enclosure liberated for this purpose. It works well with compact flash devices in a PCMCIA-CF adaptor. I have verified compatibility with a Performa 475 (8.1) as well as a beige PowerMac G3 (9.2.2). The CF cards are formatted "Mac OS Extended/OS 9 driver" with the Mac OS X Disk Utility (10.4.11). This works for an old Jenoptik 32 MB CF card and for a Kingston ultimate 1 GB (100x) CF card. Hot swapping works well (use "put away" to unmount and use SCSI Probe to hot mount). You can use it as a start volume, also. The hack for ResEdit Drive Setup to support initialising of this device in System 8.1 seems _not_ to work. (To use the drive as a boot volume for 68kMacs, format Mac OS Standard.)
Successfully tested with Intenso 4 GB SDHC card in a Delock PCMCIA PC-Card CardReader 33 in 1, Product code 87401. SDHC cards will only work in recent SDHC enabled card devices.