I want to upgrade my mid 2012 Macbook Pro 15-Inch to 8gb RAM. Where can I buy it?

I've been looking around on eBay and Amazon and I'm not sure how to buy this. I looked on this Apple site http://support.apple.com/kb/HT1270?viewlocale=en_US&locale=en_US#link2 to see the specifications but whenever I look online I can't seem to find them exact. I know OWC sells it but I was hoping to find something cheaper. If I went down to 1333 mHz or 1.5V memory would that hurt my laptop? And does it have to be the PC3-12800? Because I see RAM with different numbers. I'm new to this. If you have links to RAM that would work for my laptop that would be great. Thanks!