By Helical
United Kingdom 
35 years ago I took over my first Greenhouse. It was small but I needed some staging with three levels. I wouldn't now recomend three staging because two is enough. I built it out of 25mm square plastic tubes and connectors and it was brilliant.
I've now purchased my second quite larger greenhouse and I wonder whether anyone still uses this plastic tubing construction and if so whether they can recomend a supplier.
27 Feb, 2016
Checked around and found what you are looking for on eBay....BUT....they and the connectors are made out of metal and the only thing plastic are the end caps. Worth a look though.
28 Feb, 2016