Where do i purchase the RAM?

Most experienced users on these forums only purchase from Crucial.com or Macsales.com. Why, both only provide very high quality products, have very fair pricing and offer excellent service. In short, there is no reason to not buy from one or the other.

Which iMac? It's important to know before you buy.

You need to know that the RAM slots in Late 2012 and newer iMacs are not user-accessible and Apple with not add RAM for you. They will service your existing RAM and replace with the same amount if needed, but will not install more.

The take-apart procedure requires professional help unless you are skilled at working inside electronic devices and can afford a new computer should you ruin the current one during attempts to upgrade. The 2015 21.5-inch iMacs have NO RAM slots so even a skilled professional cannot increase your RAM.

See: Install memory in an iMac - Apple Support

Your welcome.