12 Volt DC Wiring

I was hoping to find another 12V DC source other than the one on the new relay, because the distance to run the line and the difficulty of running the line from the switch to the relay is going to be tough. It needs to run inside the bathroom cabinet and somehow get from there, under the floor into the compartment under the bed and I need to leave enough excess wiring to expand and contract with the movement of the slide in and out. I was curious if anyone knew of a way to ID another 12V DC source.