They are very common on the west coast. Not so common here in the midwest. When I was a kid, I lived in Hawaii and would go catch them all the time. I have only seen one mated pair since moving out to StL, seven years ago. I could probably order a mated pair, but have no desire to.
There is no need to move them to a QT tank as the young wont survive anyway. Best to let them feed your tank. If you try to move the female, you will most likely break off one or both of her claws, and she will release the eggs early...and you'll end up feeding the tank.
Nick