What ways are there to interface a unity game with a bill acceptor/validator? Usually this type of devices are connecting to a computer using a serial port and communicate over ccTalk protocol. But what ways exist of incorporating this type of devices in Unity?
There is nothing built in to Unity for interfacing with this particular hardware.
However, Unity can interface with the serial ports, for example with
System.IO.Ports in C#. You will have to either find a library to interface with this hardware and hook it up in Unity, or you'll need to write your own interface using the hardware support built into your scripting language of choice.