I find them on ebay but you have to check how close they are to where you live. You can find the same boards for a fiver or fifty pound so have to search the list daily to find the bargain Anyvan will transport them for about £35 upwards depending on distance if you have no transport. We have a local Free Stuff site on Facebook in South East London wher we can find them for free if lucky.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.