Ebay is the best bet, expect to wash it and remove the fake Lego, damaged Lego, aim to pay around £10 to £15 per kilo including postage. Cheapest way by far.
You can buy direct from Lego, which is usually quite expensive, their pick a brick range is now huge. BrickOwl and Bricklink are also online market places for Lego but unless you want specific parts it can get also get quite pricey. If you have a Lego store nearby most have a wall of pick and mix Lego, what’s in it can vary a lot but it can be a cheap way to stock up. You get a pick and mix style plastic cup to fill up (changing to cardboard boxes at the moment). Don’t be afraid to stack piece together to maximises what you can get in the cup.