Q: I need to upgrade my memory from 1 GB to get updated os el capitan. Where can I buy a memory upgrade for my imac?

2 highly recommended vendors for Mac Ram are OWC and Crucial

Both have tools to help you identify the Ram you need for your specific model of Mac.

http://crucial.com

http://macsales.com

Buy it from OWC or Crucial

Buy 4GB, 2GB is not enough

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing 

Be aware that, even with 4 or 6 GB of RAM, your computer still has a slower, older processor and El Capitan runs best on newer hardware. Considering that your iMac is about 9 years old, you might want to consider saving towards a newer model.