Any where that I spot one that takes my fancy really, I did buy some very nice large half barrels from Mole Valley Farmers recently, that were a good price for what they are, maybe there's one somewhere near you.
I find garden centres to be the dearest place for them, in fact I went to my local one the other day to buy some gloves only to discover that they were cheaper in Harrods of all places.
When I told someone I liked the ones in HB they said they were always over priced there and would be cheaper elsewhere
Did not like the look of any in ikea though
Usually have a ride round the the local garden centres,compere what they've got and the prices and go from there,but where ever you get them make sure they're frost resistant and give them a good examination for flaws etc,theres nothing worse then just getting them established only to have them crack during the first cold spell we get.
I think a lot of places are dear for pots, you spend £10 on a plant, then you have to spend £20 on a pot to put it in.
None of which is much help, I know.