This week I received a sheet of aluminum eggcrate from Central Lighting Plastics. It is the standard stuff - half inch grid, half inch thick, mill finish. I got a 23 5/8 inch square sheet (600 mm square) for $35 shipped from Illinois to Oregon (~3400 km within the continental US, for nonlocals), which is a lot cheaper than some of the options we've found.
http://www.thelensguys.com/AL-E1M.aspxIt looks good. It arrived well packed and undamaged.
I have no affiliation except as a one-time customer.