I think I got it from Maplin.....but it was about 10 years ago!
It was quite expensive, from memory the best part of a tenner for a pot so small it makes tins of Humbrol enamel look big (I thinks it's got silver in it which might explain the cost) but then so little is used each time that it lasts pretty much forever and it doesn't seem to dry up either - I used some only yesterday!
I'd be interested in if its still available though, and if so how much it is now!