Can I purchase regular DDR3 RAM for my MAC (not from an iMac store?) I know that buying and installing RAM from a regular computer shop will be cheaper for me. I was just wondering if a Macbook Pro 17" early 2011 model uses specific 'Mac' DDR3 RAM or if I can get away with any DDR3 RAM?
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