I've been around all the time, just not on the forum. PM me if it's urgent, say, needs to be answered within less than a year or two
Just to clarify: RTK0 is just the interface name for ifconfig, as opposed to EN0 or NE0 for the NETUSBEE and other devices. The network card must be PCI, 3,3V (has two notches in the connector) and have the right chip. Those are generic, there are only a couple vendors for the chip, but hundreds of producers for the cards.
The name of the chip is rtl8139 as is the name of the driver.
A search for "rtl8139" on ebay.com yields lots of results, many for less than $10. I don't know off hand if both the "C" and "D" variants work, should be noted in the driver readme or some forum entry. I can check later which kind of chip it is on my card.
Schneider CPC464 (long retired), Atari Mega ST4 (retired), Falcon+CT2A, Falcon+CT63+CTPCI+Radeon 9250, Milan040+SCSI+Rage