Just use an ordinary UTP cable
Connect 4 walloutlets to the cable as shown below.
Use the straight-through cable if you tap an ordinary switched network
Use the cross-over cable if you tap an PC directly connected to another network
The results may looks lik this
Place you tap (8 wired walloutlets) between a PC and the network
Connect your sniffer with two interface to the 2-wired walloutlets.
Put both interfaces is promiscious mode and start the capture on both interface