Place near us called kirton sectional buildings or similar do some nice ones and used to deliver and erect nationally for a reasonable price. Might be worth giving them a bell.
I'd hunt the local area, web search, yell.com etc etc. Ours was cheaper and far better than the DIY store ones, as said. Plus they added things to our liking like door size, window placement and size etc. Installed in a few hours cheaply too, we already had a pad though.
Should be a decent-priced BS-free local place (seem to be some posh shed places these days with posh prices to match...)
As it happens, that's exactly the size and construction I'm considering as well, except I was thinking ply or OSB on the inside and shiplap on the outside.
Out of interest what did you do for the roof construction? I'm looking at a 30degree pitch on mine.