billor,
The plastic water stop that you describe is a "Z" shape piece of plastic whose top half is embedded in the deck. It comes as a part of the cantilever deck forms. Normally, the styrofoam forms are boxed with the waterstop and aluminum wire. Waterstop is not normally sold separately.

You may be able to find a concrete contractor who uses these forms. They will be your best bet to find a small piece. If you are trying to replace the entire waterstop, you are on a fool's errand. Small pieces can be glued on with some success. Your pool tile contractors are correct.

Hi. Thank you for the prompt replay.

One leg of the "L" is 1/2" long and lays over the front face of the tile. The other leg of the "L" is 3/8" long with a 1/8" flap that acts like a lock to keep the "L" locked into the very narrow space above the pool wall behind the tile and the under side of the cement deck.

The small 1/8" flap at one end of the "L" forms a water seal between the top side of the pool wall and the bottom side of the deck.

Imagine an upside down "L" at the front face of the tile and the upper edge of the tile extending between the lower edge of the deck. It appears the tile, which is cemented to the pool wall helps hold the plastic against the lower side of the pool deck. Bill