In the past when I've found egg crate it has primarily been at local hardware stores. There are a few fish stores though in my area that carry it as well. Tends to be a little more money but that might be a place you might find it.
use eggcrate and spray fome to make a background and land area. put rocks in drift wood in the fome wile its seting up. and the leftover area of fome use colored cement or use cilacone and put coco fibers over the wet cilacone. try youtube!
I used light diffuser grid (plastic) covered with fine mesh and glued on with aquarium safe silicone. I made a whole ledge and caves but it still didn't limit my water flow. I could use a vertical box in the corner to hide my tubing and power heads. I used silicone to glue aquarium plants and rocks or decorations on.
