Tekonsha's web site for the P3 is still not as complete as for other models. If you look at the
Prodigy page, you'll see the usual level of documentation, including a link to their generic
Brake Control Wiring Instructions. As described there, the circuit breaker is in the line from the battery positive terminal to the control's power input lead (black wire).
Tekonsha specifies "20 amp for 1-2 axles, 30 amp for 3-4 axles"; with only one
axle, 20 amps is more than high enough, but if the wire gauge is appropriate it is harmless to use the 30A rating.
I actually bought mine at an RV dealership, but they're common automotive
electrical components, and only cost a few dollars.