I too have never seen a lock for the cable, but if your intent is clean ends, Get a 7 pin connector SHELL from any auto or RV place (no connector just the shell and door( and install it in the trailer.. If a 5er I'd put it so it's on the underside of the "lip" where the king pin is. and plug in there.
if you wish to lock it, a padlock and bracket are all that are needed, If you need I can design something but basically....
A "L" bracket (A wide one) from Lowes or Home Depot et-al, with holes for the padlock shackle, and set it up so the padlock goes just below the plug preventing it's removal from the shell.
Or yoy could use s simple eye bolt Just replace one of the 4 shell mounting screws with an eye bolt and nut (to tighten to the shell) But I'd put a bigger lag-type eye bolt next to the shell, make sure the padlock prevents removal of the plug from the shell.