Where should you purchase?
One of the main places you can purchase products for your computer build is Amazon. They offer affordable pricing and have much of what you need to complete your build, often provided by the companies themselves.
Newegg is a website that tailors to PC building and is an all-purpose technology website. They offer everything you need to build a computer and have great pricing. They often have deals on many products that include discounts or some additional product with a purchase. If you can't find what you are looking for, they often have it in stock.