Check out your closest Truck Stop...
One of the best places to find remote speakers for your radio is your closest major truck stop... the normally stock all kinds of stuff for the CB crowd.
They work fantastic and the price is right.
I have two mounted under the seats in my truck, I just put some "hook" side velcro on the back and press it into the carpet under the seats.
My Kenwood radio is a dual band unit with dual speaker outputs, so I put one side thru the passenger side speaker and the main band side through my side.
Works great, the separation lets me know which band is active this way.
You just have to be careful to route the wires away from the slide tracks for the seats, or they might get cut when someone adjusts the seat forward or back.
Install is really quick and the velcro seems to hold them tight even on the roughest roads.
Prybry