Newegg, Amazon, etc. As title says.
I like to purchase my stuff from Amazon.com because there are a lot of shipping options and their return system is great. They automatically placed an order on an item I wanted replaced before I could even return it to them. They are similar to Ebay but seems more secure. Prices are good too. Not to mention no tax for Californians
I have ordered stuff from newegg before but there are usually less shipping options for me and there is tax. So Amazon is usually a little cheaper. But I have never had any issues with them before