I reused a pre-existing cord from the old irrigation controller. I believe you can purchase a "wall hugger" style outdoor or heavy duty extension cord and cut to length.
A good trick the previous installer did was to coil the sprinkler wires instead of cutting them to length. This gives you a lot of length to work with in the future without taking up too much space. I strengthened them out during replacement and coiled them around a thick screwdriver for the final install. It's coiled in both installs but you can see them clearly in the second picture.