After many moons my tumbler medium has finally stopped working effectively, but to my horror, looking on line, I see that I must have been asleep since I last bought the stuff.

FIFTEEN POUNDS!!! x 100

I recall that last time I bought it it was in a HUGE plastic bag and cost about 1/6d. Was a while back, though...

So where's the best place to get it these days, and by that I mean, of course, the most economical?