The suggestion from Crossroads is great for small motors, but if you want to drive an RC sized motor you will need an RC Battery for power and an electronic speed controller from the hobby shop. The good news is that RC Battery technology and speed controller technology has changed a lot in the past few years so you can get good kit at a very low cost on internet auction sites.
Look for Tamiya TEU101 or TEU104, these speed controllers are supplied with Tamiya radio controlled car kits and are almost always changed to something more powerful. Anyone who has been in RC for any length of time has a draw full or these and will let one go for a few dollars. They can be used with Johnson 540 motors which are again supplied with most Tamiya models and then changed for something more powerful, these motors should be a dollar or two used.
For batteries, everyone is now using LIPO batteries, so there are thousands of unused but perfectly good Nimh and Nicad batteries allover the web.
If you can find an RC Racing club there will be someone who can give or sell you all you need, battery, ESC, 540 Motor.
So depending on the power you need go with Crossroads suggestion or look out for the parts suggested above.