Hi,
Lowes or Home Depot in either the lighting or drop ceiling area, but don't ask for egg crate it is light diffuse panel or something like that. Normal use is to use as a drop ceiling to refract the light of the fluorescents.
I just cut mine to size and placed it on the edge where the hood would have sat. I only have a 55gal so I didn't need pieces that large, but egg crate comes in panels the size of drop ceiling panels so they are pretty big.