The best way to buy Lego is just to buy bulk dumps from families that stopped playing with them. It's going to be piles of unsorted bricks of all kinds and that in itself is a stimulus for creativity.
Just go on ebay or the local equivalent and search for 'pounds lego' or 'kilo lego' and you should be all set.
Just go on ebay or the local equivalent and search for 'pounds lego' or 'kilo lego' and you should be all set.