Can I purchase regular DDR3 RAM for my MAC (not from an iMac store?)
Generally in principle yes. You do not need to purchase expensive Apple memory.

However you will need to check for each model - the best thing is to go to a reputable memory supply company website which sells certified RAM and look up your model.

You can search for them online but here is a respected store (I have no affiliations with)

http://www.crucial.com