I've built a small (6") airboat with legos and added a plastic propeller from an old rc airplane.
I need to run the propeller with a battery and a switch. Please help

Here is a photo showing a battery, switch, and what looks like a little motor.

Back in the day, all these parts were available at Radio Shack.

If you want to control it, maybe look here:
Hobby-Lobby's RC Catalog of Radio Control Models, RC Airplanes, RC Helicopters, RC Boats! (http://www.hobby-lobby.com/)
Here's what else you can do with Lego's:
http://www.metacafe.com/watch/yt-RcEpaGjBowM/the_complete_lego_pulp_fiction_edited_for_time_not _content/