you can get it locally at Port plastics in Portland. Thats where i got mine. I actually use a 3/4" thick piece, then i drill 1/2" holes in it to hold my acrylic rods with teeth in them that grip the rockpi9le like glue. I drill the holes 1/2" deep into the 3/4" hdpe, the rods sit flush and can get a clean sweep of the floor. Never have to siphon anything.
I also stagger the rods pretty tall(like 8") in places so it makes a hollow reef, makiong even less flow restriction. I figure if I cant see the rock, i dont want it there, it just gets in the way.
I at first didnt like the starboard, but as soon as it gets covered in coralline it kind of grew on me and i like it now.