It's Rubbermaid's Fast Track System; I'd simply call around and ask my favorite local hardware or big box store if they have it in stock. You can also order it online here:
Could I have this on the wall (fence side) and then where the blue tubs are have a low line cupboard that we can keep random bottles and the like.
(Also is there a way to raise the roof in only this spot? Might be necessary to fit the ladders in)
http://www.diylife.com/2010/10/08/rubbermaid-fasttrack-review/