I also buy some hair products from hairdressing shops. They offer fairly cheap hairdye for non-standard colors that regular stores often don't carry, or only have products that wash out very quickly. And I get conditioner by the liter (33oz), which is a little inconvenient, but it's a huge steal for a better quality than the usual pretty packaged consumer products which often include harmful ingredients.

Specialty stores in general are often the way to go!