The safest way to get the driver is from your computer or motherboard manufacturer's website (e.g., HP, Dell, ASUS, MSI, Gigabyte).
If you truly have an old RTL8139 card (PCI, not PCI-E) and you need Windows 10: realtek rtl8139 driver 810x family pci-e gigabit windows 10
Cause: Windows Update is overwriting your driver. Fix: The safest way to get the driver is