HMV have a very limited selection, but I have seen them stock both DVD-A and SACD. I usually buy from either: www.play.com www.cd-wow.com
both are very reliable - play.com delivers at the speed of light, whilst cd-wow is useful for DVDs at silly prices (and CDs! - or at least they used to be until recently when regulations forced them all to charge UK consumers more booo ), usually from region 1,3 and 4 in NTSC.
Thanks I've used play.com a lot in the past and likewise CD-wow. Since those new regulations came in, they must have been cramed with lastminute orders as everything I ordered last week is now on back order!