By the time you have managed to track down a usable supply of free planks you may as well go to a merchant a get a job lot of 3 meter decking boards and use them.
They are pretty rot resistant but you will have to double up on them as they are 150mm max tall.
I have never seen a cheap supply of scaffold planks as they are good quality planed construction grade timber, even dead ones are sold on as reclamation so I can't see them being that cheap darn saarf among the chattering classes with their gardens and allotments.