I got the totes from Big Lots, but Target also sells them, tho both places tend to have limited stock that quickly vanishes - Sterilite Large 2 Layer Stack & Carry Clear With Green Accents : Target. You can also order them on Amazon for about twice the price.
However not too long ago I built a bigger, better, more mechanically secured tote and talked about it here: Magnet-Ready Army Case 2.0
EDIT: Just saw you specifically talked about steel sheet. I got mine from Lowes in both cases, but any American big box hardware probably has them. I wish I could tell you which gauge I got but I didn’t take note 