My understanding is that the cable is a standard phone cord for a telephone handset. You should be able to pick one up at any store that sells phone...
The first computer I ever used was a TRS-80 Model I Level II (Upgraded from Level I). Currently, I have a nice collection of TRS-80 Pocket computers and a few Model 102s. I recently put together an Altair 8800 reproduction. I've spent most of my career on the software side of things, but now I'm getting deeper into the hardware side.