It's been a long time since I've spec'd cardboard but if I'm remembering right E-flute is the 1/8" stuff with the fairly tight corrugation.  If you're talking about doing life-sized standies I'd think it was too thin.  Many years ago we got a trailer load from Rock-Tenn in Georgia, then we found a local company that makes boxes that would order it for us as we needed it.  I know you can get cardboard sheets from U-line but I don't know the grade.
Personally I use corrugated plastic, due mainly to it's availability.