TI has designed the LaunchPad to be powered in two ways. The most straightforward is via the USB connection. This is how most experimenters will use it. Your computer puts out regulated 5V on the USB cable. The LaunchPad converts this 5V to 3V to power the microcontrollers on board.
If you want to power the LaunchPads on your layout this way I would suggest that you get a USB powered hub with a lot of output ports.
The powered hub has a wall wart transformer that plugs into the hub and supplies power to the attached USB devices. Therefore, power is available even if the hub is not connected to your computer.
Here's one from Newegg (a popular on-line retailer of computer products) 7 USB ports and capable of supplying 2 amps of power (enough for 20-30+ LaunchPads) Powered Hub.
Then get some USB "Male A" to "Female A" extension cables (sometimes the dollar store has these, if not try All Electronics). Use the extensions to connect the hub to your LaunchPad's USB cable. Now your Launch Pads are powered with no muss, fuss or bother and the cost is not great. Non-powered hubs can be cascaded from your powered hub to feed more LaunchPads (see diagram). Not a bad option if your layout is not too large.
If your layout is really large or the distances between Launch Pads is long, you may want to run a power bus for the LaunchPads. This bus must be clean, pure, regulated DC - the output from a spare power pack will not do. The voltage on this bus must be between 3V and 3.3V and cannot exceed 3.6V.
The Launch Pad documentation tells you which jumpers to disconnect to use external power. Look at paragraph 4.3 in this document here. Apparently position 5 (labeled VCC) on jumper J3 must be opened; then 3.3V can be applied to J6. Do not have a live USB cable connected when external power is used. Of course, your LaunchPad must be programmed before hooking up to external power.
A good power source would be a converted PC power supply. Read how here. You may be able to find a wall-wart that puts out 3VDC, but it has to be regulated. Another inexpensive option, that I have not tried, may be the Philmore MW122A power supply (about $25 but available from MCM Electronics for $19 and when on sale for as little as $15). This looks like their old battery replacement unit but they claim full regulation and low ripple. Whatever you use, maintaining regulation over very long runs can be a problem.
Depending on what you connect to them, these LaunchPads do not draw much current and a couple of amps should power many.