In Europe, that would probably be Aldi/Lidl.
In the U.S., maybe Costco/Trader Joe's.
For online, CamelCamelCamel/Amazon. (for health/beauty/some electronics but not food)
If you can buy direct from the manufacturer, sometimes that's even better. For example, I got a particular brand of soap I love at the soap's wholesaler site in bulk for less than half the retail price. For shampoo, buying the gallon size direct was way cheaper than buying from any retailer.