Ethernet cards and tcp/ip stacks are available.

ip65 (https://github.com/cc65/ip65) for the 6502 supports 3 chips, 6 cards, and 3 platforms (Apple II, Atari, and C64).

For under $5 you can buy a wiznet chip that has an onboard hardware tcp/ip stack.

Well, you could hook it to an AppleTalk network w/ stock hardware and then setup a Mac gateway to get it onto an Ethernet network. I think I've got a setup for that around here somewheres...