I got mine by posting a WTB ad here. The seller was helpful and responded to my request to include a floppy with HD SC Setup (patched version).
There are other types, such as LocalTalk -> Ethernet, SCSI -> Ethernet, and of course the PDS slot. If you get one without a RJ45 style connector you might have to fiddle with either a serial adaptor or a ThinNet adapter, which requires tedious termination and setup procedures; none of which I am famarilar with because I decided from the start to get an RJ45 card -- the other protocols which were swirling around in the networking pool back in the day like TokenRing, ThinNet, ...and so forth never quite took off -- but Ethernet did.
Heck I just checked eBay right now, and
this shows up. They are common, but also not so common because the only people really left with them are enthusiasts. (Just like old BMWs, Porsches or vintage cars...or anything like that)
Don't forget that the IIsi also has a 030 PDS slot, and it is compatible with the SE/30. If all else fails, you could look at a NuBus adapter and use a NuBus card.
They can vary in price, be prepared to pay about 30-50$ for them. They are worth it -- floppy disks get old real fast.
Unfortunately there isn't really a "USB Flash Drive" in the old days like today -- they are really very very useful.
Actually the one you see there is probably better than mine -- it has a 90º slot for an additional card, for an accelerator (up to 50MHz), color card (you'll feel the performance hit, we had one of these things hooked up to a 13" back in '95 or so) or other things.