Here is a very simple adjustable power supply I built for powering my projects. If you're into electronics sooner or later you're going to need one. This particular design is very easy and cheap to make. Plus it's based on a ready made module which simplifies everything even more.
LM2596 module (plenty of them available on fleabay - average price just over 3$)
AC adaptor (preferably one that can output at least 24V, but not more than 35-40V)
Some wires and male headers