The best I have found so far is from Thorntons as it comes ready to use without having to add the oil. Its a little more expensive but works brilliantly and effortlessly in chocolate fountains and fondues!
My daughter had one at her last birthday party and we just used cadburys dairy milk, it takes more than you think and you have to add a little oil but worked so well that we are doing it again this year. My sister got choc for hers from Lakeland (a mail order co) and I hear that Galaxy choc is also good. Hope this helps.x