24v transformer supply
Yes, you can use a common 24V 40VA transformer that mounts on a junction box lid.  If you have a handy junction box location already built into your building, You can simply mount it there.  Or you can add one to your building using legal wiring methods.  

I am not beneath getting  a 3-prong line cord (often a 3-prong extension cable with the socket lopped off), and stopping by the local electrical supply house with the line cord in hand to acquire a proper strain relief for that cord and a nice stout aluminum junction box while I'm there. Also a 10-32 ground screw.  I'd bring the transformer too for fitment, and that isn't rude since they typically don't sell transformers.   Then, mount and strain relief the extension cord, ground it to the box, and hot and neutral to the transformer 120V side.  The 24V terminals should be on the outside of the transformer.