Rubber Pad...
I have one of the pads you're talking about. It is about an 1/8 inch thick rubber pad that has holes patterned for 15 balls to "sit in". The holes in the pad are such that 2 1/4 inch balls sit "frozen" in place. Once positioned correctly on the spot, you can tap the balls into the cloth to set the racking of the balls to be nice and tight. This is the same effect you get using a Sardo, marking the table and then pressing the balls into place. This pad works great but it is only as good as the person putting the balls back in the same set up, i.e. not shifting the whole rack forwards or backwards.
A friend gave me the pad, and he said he got it from a billiards supply in Virginia I think. Maybe it was Billiards Direct or something like that. He said it retailed for $20. I like using it to "set" a rack area up on new cloth but as soon as people start moving the rack around you lose the set.