How do I purchase ram to my iMac?

Al mentioned OWC which you can find here.

https://eshop.macsales.com/shop/apple/memory/

Some models of Mac are quite easy to change memory.

Have a look at the videos on their site showing various models with memory being upgraded.

Not all models are shown.

iMacs are easier to do than laptops.

First you need to know the exact model of your Mac.

Go here to find the Mac System Report with all the details:

Then locate a reputable dealer, such as OWC (Other World Computing).

The dealer can advise you IF it is upgradeable and HOW much is the maximum.

One thing you should NOT do is cheap out on RAM.

As they say, "if you want something good for nothing, you will probably get somethin' good fer nutin'."

Al