I really need to build a box like this. I've looked for that non-galvanized expanded stuff before at HD with no luck. What section is it located in?
My HD didn't have any, didn't even stock it he said. Lowe's had it but it was 12x24 so I went to my local ACE Hardware and they had the 24x24 sheets. Lowe's and Ace had it located by the nuts,bolts, and rebar section.
Edit: If you find the proper size at Lowe's (assuming you go there), the angle iron you get needs to have the white paint on the end and the sticker should say STL which stands for stainless. I had to figure that one on my own though, they were no help at all. Hope this helps.