Though by RGB do you mean SVGA or Composite? You'll need to know which you have before you can get the proper device.
Anyway, what you want is a basic A/B switch, and your best bet at finding one would probably be your local Radio Shack. I would strongly recommend getting one that has manual buttons to push in order to switch outputs.
You could probably also use a KVM switch to do this as well. Just ignoring the keyboard and mouse hookups. Though a KVM switch is likely only to work with SVGA. If you need Composite, you'll have to go to Radio Shack or a similar sort of electronics parts store.
I have an nVidia PCIe graphics card capable of handling two monitors (one via DVI and one via RGB). I currently have a 17" DVI monitor hooked up, as well as a 15" analog/RGB monitor. I recently purchased a 32" LCD HDTV, which has an input for RGB and can be used as a monitor.
I have three monitors, with only two outputs. The 15" monitor and the 32" TV are in different locations, so I would use them for different things.
To easily switch between the two monitors without losing much quality. I need to be able to output 1366x768 @ 60 Hz to the LCD TV. I don't want to switch the cables to the output all the time. There MUST be some sort of inexpensive switch out there which will accomplish this task.