the national trust opperate a sawmill at ashridge estate (just over the border into herts) but i'm not sure about the drying - you will be lucky to find a timber yard with a klot of kiln dried timber stackked up where you can poddle about.
and Mackrory & Sons who are on the nettlebed side of henley (actually oxfordshire not bucks but only just - phone 01491 642033
it also occurs to me that you might find something through reclaim - the only one i know of in bucks being P T Demolition Ltd Tel: 07702 212960 who i think are near wycombe (tho i could be wrong about that)
plus of course there are hospice warehouses in both ayesbury and milton keynes who sell s/h furniture - you might find a dresser, wardrobe, book shelf etc from which you could recycle some timber.
i shall give them all a run around and have a look i suppose i am looking for the old timber yard my dad always seemed to go to when i was a kid however that was in essex and it is rose tinted goggles and all that eh