I bought mine in Hobbycraft - it's the Pebeo/Gedeo brand and they sell either kits with moulds or the two larger bottles (which I bought) of just the resin. It's very easy to use, very clear and very tolerant of beginners! The best news is that it's one of their top lines so you should be able to find it at any branch. If not, it's available online from http://www.truroarts.com/modelling-sculpture/materials/cat_474.html
(they also sell another brand which looks a bit cheaper).