There are 2 locks you may be refering to...
A receiver pin lock - so someone doesn't steal your 2 inch square receiver hitch & ball. ( with or without TM attached)
A coupler lock - so the trailer can't be lifted off of the ball.
samples:
http://www.amazon.com/Master-Lock-28...719185&sr=8-19
They are sold at Camping World, Walmart, Kmart, etc.
Watch demo:
.