You can get 1.5(16 guage) box in Heaton Buckley near Newlands cross. I got some 19mm and 25mm a while back.If you are in Dublin, you can get the lighter box section steel at tubecompany.
It's not on their website, ring them, It was the only place in Dublin I could find 1.5mm thick (16 gauge) 25x25mm and 40x40mm box section
http://www.hevac.ie/