PeterTaylor has seen a handful of boosters in WHSmiths?, next to the tills nearest the entrance.
http://www.blackborder.com: they're likely to have among the best prices you'll find outside of eBay, because they survey 15-20 good online card retailers every few hours and match their prices to the best out of them. AlexChurchill has bought from them a couple of times with no hitch.
eBay: As trustworthy as the person you buy from. Which varies a lot. AlexChurchill has had mostly fairly good results from British, American, Canadian and Australian sellers. Look around for those with cheap shipping to your country. He's had at least one bad experience from manaleak.com and two different German sellers.
Other websites that AC's had good experiences with are millionpointfireball, wagicalplace, and others.
I know of one place in Canterbury that sells them a bit cheaper than mentioned above. Westgate Games has boosters for £2.40, and tournament packs for... £6.99, I think. Probably not any real useto most people, but it is cheaper than most places. --FR