Tesco at Spytty were out of most stuff last weekend, and not sure when they would restock, the advice I got was to order on Tesco Direct and then collect in store, but on there it's all full price.
Wilkinson in Cwmbran is OK, but they are reducing what they carry slightly, still got the important basic stuff like fermenting vat, keg, one and two can kits, plus a few additives and some yeast when I was there last weekend (rumour had it they had a sale as well, and I needed a big funnel).
The Homebrew Crew in Roath, Cardiff is excellent, been there a couple of times and not caught them out yet, everything from a spare S30 valve (they exchange cylinders as well) to dried hops, malts and all types of yeast under one roof :)
They were shut for a while as a car crashed into the shop, but it's all fixed up now and open all day Saturday :)