Cantref is closed for fishing at the moment, due to the dam being repaired. I believe that it's closed until next season.
Llwyn-on, just below Cantref, is also now closed for fishing, due to low water levels.
The fishing permit machines are in the car park of the Garwent forest centre.
I buy my permit online before going, and you can also get updates of when it's reopen for fishing from here :-
The Booking Office - The Wye and Usk Foundation
You could try Usk Res. or Llandegfedd instead.
Welshfish.