Any local pet store that sells fish! I always found large garden centres which had a pet centre to be the best bet. I love chocolate orandas. Such pretty goldfish. Buy them small because they're cheaper. In the right environment they'll grow to a good size.
you could try Pets@home they sell them about �4 each I think it depends on which type you get they sell red caps, chocolates, orange, black moors and they sometimes even have lion heads but these really cost
I don't know if you have kept them before but I had 5 and over time have lost 3 because of swimbladder problems. Two I bought fairly small and three were quite big when I got them. All came from a garden centre pet store.