If you need to keep it cheap, then the pre-build modular approach does work.
Here are some examples:
This module is pretty decent for ~$20
and building it in in a box:
Whatever you do, make sure you get some form of current limiting, it saves a lot of expensive magic smoke release. Obtaining the raw power from a laptop power supply (typically 19v @ 2A) would be a good solution and avoids the hassles/risks of mains wiring.