A relative came to me today needing help connecting to the internet, so one of the first things I did was obviously check to make sure there was an ethernet cable connected.
There wasn't. In fact, there was no network card, so I opened the case up because I had a network card in the car.
Unfortunately, all the PCI slots were filled, and all that was left was a 16-bit ISA slot.

Does anyone know of a place online that still sells ISA cards, or should I start looking at garage sales?

Good luck, you'll probably have to buy them used.

After digging through my attic, I found one. No telling if it still works or not, but it's worth a try.

Ancient iX1 Micro. It's a bit taller than most network cards, and about as long as an early Radeon, maybe longer.

Is one of their pci slots taken up by a modem or something they don't need?

One is extra USB ports (only had two built in), one is a sound card, and one is a serial port for a fax modem.