It is worth looking on ebay under "gunstock blanks" - I have picked up an "exhibition grade" rifle blank from Australia at very reasonable price - they are air dried and mine was cut in 2012 and I have kept it aired for over a year to allow it to aclimatise - the grain flow and the figuring are outstanding and it is close grained and dense. Their prices include postage.
I have also bought a couple of highly figured blanks from USA - one in Claro walnut which is too open grained and one in Bastogne walnut which I will use as soon as I can get my cataracts done (I am loathe to start any checkering until I can see sufficiently well again to checker at 22 lines per inch). The Claro blank will need grain filling and oil finishing BEFORE checkering as the tips of the diamonds may otherwise chip.
I see there is someone in Slough who is selling walnut blanks on ebay but they seem to be mainly for shotguns.
When buying a blank I avoid those with a serial number starting KD - this usually means they are kiln dried.
Hope this helps