'found an old Realtek RTL8139D PCI card whist clearing out old computer stuff. It's an ancient Connectland CR-CNL-FAST-R model dating back to 2008/2009...
http://www.connectland.eu/products/fiche/category_id/133/culture/en/id/167/name/10-100-mbps-fast-ethernet-pci-card
Gave it a shot in my old Vostro200 running High Sierra and Mojave. Installed the old IONetworkingFamily fat-binary kext from osx86.net and the card still works under macOS! First drivers for this card were released for OS X 10.1/10.2!
IONetworkingFamily.kext.zip
Note that the card is not supported by Mieze's RTL810x FastEthernet driver.
Initially, I only installed the RTL8139 kext in /L/E and that brought the card to life but no traffic passed. The card only fully worked once I actually installed the entire IONetworkingFamily kext in /L/E, without touching the vanilla equivalent in /S/L/E. There must have been some dependancies... After the usual permissions repair + cache rebuild + reboot, the card was 100% functional. There goes an old 32bit PCI (not PCIe) card in macOS High Sierra and Mojave! Must work in all previous versions from Snow Leopard to Sierra too.
vostro200:~ admin$ lspci -nn
[...]
00:19.0 Ethernet controller [0200]: Intel Corporation 82562V-2 10/100 Network Connection [8086:10c0] (rev 02)
[...]
04:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL-8100/8101L/8139 PCI Fast Ethernet Adapter [10ec:8139] (rev 10)
Edit: 17 Jan 2021
osx86.net now dead. Old link to file was: https://osx86.net/files/file/3172-realtek-rtl8139/