Wood for cladding inside of shed (Netherfield, Kendal LA9)

Our son has got a bit of a project to try and convert a shed into a make shift sauna. The plan is to line the shed with tongue and groove but it's expensive to buy so is thinking of a patchwork of any clean wooden planks he can get hold of, or even wooden doors. It's important that it is actual wood and not laminate. Any help gratefully recieved. We can collect and are based in Kendal. Thank you.