Electrical part question
There are a few names for these type of connectors:

DC Connector
DC Power Jack/Plug
Barrel Power Jack/Plug
Coaxial (or Co-Ax) Power Jack/Plug

Examples of parts you can get for making your own adapter cords or soldering to a PCB:
DC Power Jack:


DC Power Plug:


Regarding the size of these connectors, this excerpt from Wikipedia says it best:

The most common plugs are 5.5 mm (0.22 in) in outside diameter (OD) and 9.5 mm (0.37 in) in length. Two pin sizes are common in the jacks for this size plug body, 2.1 mm (0.083 in) and 2.5 mm (0.098 in), and the plugs should ideally match. Generic plugs are often named for the pin diameter they are designed to take.
[...] The 'tip' (i.e., the inner conductor) usually carries the positive (+) pole.

You can find these at online electronics vendors (DigiKey, Mouser and All Electronics) as well as Radio Shack if you happen to have one nearby.