Any suggestions as to where these can be had very cheaply (can be damaged stock)?

Bamboo canes
Terracotta pipes
Catering tins
Softwood battens

I’d say you need to find free things that work rather looking for specifics. If it is a school thing could finding stuff be part of the project? This was at Chelsea a couple of years ago. It was Knightsbridge School that made it. Old shoes, Tyres, oil drums..

http://www.diy.com/corporate/community/waste-donation/

B and Q above try them.Also local builders merchants.

Pallets can be a really good source of wood and would be ideal for batten material, also the blocks are prefect with holes drilled in them for insect homes.

If you can find pallets marked or stamped with HT it means they were heat treated as opposed to using chemicals as their preservatives.

Idea stolen from that madman Colin Furze on youtube, in the background of one of his videos there were old pallets that had been stained and had growbags slotted into them before being stood up against the wall. The plants grow out between the planks and can look quite good.

chat up the council/local authority/local civic amenity site (*dump in old peoples terms) and get permission to hang about intercepting and taking (which may not actually be possible)

ask on freecycle
local paper adds etc