where can i purchase memory for my mac book pro 15 inch (mid 2010)??

Apple does not manufacture RAM - they use third party RAM in all of their models. And, Apple retail stores do not sell RAM - the only time to buy RAM from Apple is when you custom order a device. The two RAM vendors mentioned are the two most recommended here and test and guarantee their products with Macs.

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

http://www.crucial.com/usa/en/homepage

double check to see if the ram you want to buy is compatible with your macbook https://support.apple.com/en-us/HT1270

good luck!

I suggest that from an Apple retailer for most reliable, durable memory units .