Apple memory for upgrade purposes

Upgrading my current laptop is not the best option for me at the moment especially since I am financially strapped at the moment. But since my mac book is getting slower and slower due to the necessary applications I have to run on it, the next best option would be to simply get an additional Apple memory as an upgrade for it. Usually laptop memory is the only thing essential to make the laptop run faster and smoother as this determines the speed and capacity or running many applications. Looking for the correct RAM memory was easy but I was told to limit my choice to those original apple memory as this is the only thing which runs in mac book properly most people say . Having no choice in the matter, I decided to finally purchase the upgrade memory for my laptop.