Tony writes about the innards of 10-base-2 Ethernet transceivers:
The power supply comes from an isolated DC-DC
converter in the transceiver case which takes in 12V from the computer
and gives out -9V for the transceiver chip.
I'm not sure about separate transceivers, but the ones integrated on
NIC cards for PCs often used a little non-potted four-pin DC-DC converter
module made by TDK. At a guess, it's about 20 mm by 10 mm. I've successfully
kludged them onto boards that were designed for the potted module style.
Anyhow, I have a lifetime supply of NOS of these TDK converters, so I could
sell some cheap or trade them for other small stuff.
Eric