Hello, here are my answers, sorry for the delay but I have not been visiting this topic for quite a while now ...
Unharvested maize plants were available some years ago at www.witomo.de, made of flexible plastic from a manufacturer called NEW RAY
Iceland moos does indeed find its use a potato haulm (brown variant), carrot tops (light green) oder beet tops (dark green) in my dioramas. They are available at a dedicated train modellist shop not far from my home ...
The tubes and wires I use on some mashines ar either wire for bicycle lighting systems or dedicated 1/32 scale crafting wires available at www.witomo.de for instance
(Warning) stickers are available at www.witomo.de or at www.agrartoy.de, these are at least my sources and they are absolutely trustworthy and reliable. Dont know if transfer might be too expensive to the UK though ...
Regards, sascha