Kingston sells kits. I dont know who is selling them as single sticks, but what you want is a 2 pack kit. Because memory sticks are very technical things, and the CPU needs one single set of settings to use with every memory stick you plug in. The 2 pack kits have been tested to make sure that your CPU will be able to talk to them using the same settings.
Buying multiple sticks of memory that are not in a tested by Kingston at the factory kit, is just asking for headaches... Very difficult to resolve headaches.