I use two timber yards.
Nigel Fleckley at English hardwoods (Geddington Sawmill, Geddington, NN14 1AJ) for locally sourced English timbers (oak, beech, elm, ash, sycamore etc) and also bog oak.
More locally to me in London, I use E Roberts & Sons (Timber Yard, Ash Wharf, London, N18 3AB) who...