Red Monkey is the best online place
http://www.redmonkeycoffee.co.uk/
As for supermarkets, it seems to vary greatly, but some of the fairtrade specials are really interesting. I think I'm on a cafedirect kilimangaro(sp?) at the moment which I first thought was a bit rough for a medium strength coffee, but after having so many 'smooth' coffees elsewhere I'm beginning to appreciate its character a bit more. Union hand-roasted packs are quite nice too.