Bought season 3 yesterday at FOPP in London (Covent Garden area, near the 7 dials). They also have branches in Leamington and Cambridge that I know of.
They seemed to have most seasons available - for those trying to complete their original collection, these are definitely not the new `M-Lock` style they are the full 7 part digipacks.
Still at Amaxon for the original £60-£70, so £25 each looked pretty good.
Go to `about` to find store locations. FOPP