I need help finding a part for my bookcases - it is like a shelf peg for resting shelves on, but it is a rail that connects into the front hole and back hole horizontally on each side of the bookcase, and the shelf actually slides onto the rail. The shelves have grooves cut into the sides to accommodate the rail. I have found a huge amount of pegs for sale, but I can't seem to find a rail.

Any ideas? I have tried searching rail, bracket, bar, beam, pegs, sliding shelf, etc.

Thanks!