Stickers Borgin and Burkes & McHavelock’s Wizarding Headgear

The most common way to get all the bricks is to buy them on Bricklink. You can upload the partslist to your Bricklink Wanted List, Bricklink will then find the cheapest way for you to buy all the bricks you need.

You will get the digital instructions (PDF) and a partslist (XML).