Not so much live beer available in the bottle, although on draught live beers are easy enough to find in the UK. Here's some "bottle conditioned" beers I've located for various countries in case anyone's interested. The list for the UK is only a sample: there are quite a few others. The US has a lot of local microbreweries these days, many of which are almost certainly offering natural beer in the bottle. Northern France and Belgium have some bottle-conditioned beer. The larger bottles can be spotted by their champagne corks - those beers, some of theme brewed by Trappist monks, tend to be crazy strong.
Hogsback T.E.A. (widely available in supermarkets)
Hopback (several beers, including the well-known Summer Lightning)
Hepworth (Prospect beer is live in the bottle and also organic. Some branches of Waitrose)
Sierra Nevada (Their live Pale Ale is widely sold in the US and exported too. Other beers may be live.)
Hair of the Dog (This brewery brews organic beer and it says some is live)
Schoune (some Belgian-style beers, some pale ales.)
Finlandia (Finlandia Sahti Ky - cloudy traditional beer flavoured with juniper berries)