Greensand’s
Search for 25 mm or 1″ square tube systems. Old style design connectors are still compatible. I broke down some fairly extensive racking an salvaged a lot of connectors and tube salvaged for re-use. The tube ran out before teh connectors.
I bought my last couple of lots from Richardsons :-
https://www.richardsonsuk.co.uk/25mm-square-tube-system/p47?gad_source=1&gad_campaignid=40135728&gbraid=0AAAAAD8ShWz9A37gcl5cxlHtz__Q_1fzw&gclid=EAIaIQobChMIs9C-44uyjwMVRZFQBh3w4gpbEAAYASAAEgJ4fPD_BwE
Comprehensive range in both steel and alloy, quick delivery and acceptable price.
Metals4U are another supplier but their connectors are apparently only intended for aluminium box section. They do offer the smaller 20 mm range tho’ and list 6 metre lengths.
https://www.metals4u.co.uk/
Various industrial suppliers such as RS Components sell the stuff at professional prices.
Official material size appears to be 25 mm square by 1.2 mm wall thickness these days.
The finned aluminium section is very handy for quickly making strong, good looking doors with your choice of filler panel. Last time I found a very shallow chequer plate style alloy sheet that looked great whilst still being much stiffer than plain sheet. Unfortunately dumbo has mislaid the supplier details now he wants to make more doors.
Pity the finned alloy box is only made single sided. I have a really creative use for a double sided version if one could be found.
Clive