Need a tap on the cheap? I found a blog post detailing making a cheap passive tap using some wiring plugs. Makes sense! The post didn’t mention it, but, while there are 8 wires in an ethernet cable, 4 of them are not used. If the cable is built to standard, the brown and blue pairs are not used. The oranges take care of traffic in one direction, and the greens take care of the traffic in the other direction.
This is easy enough that it really should be added to the list of tasks all security neophytes should complete.