From 9to5Toys.com:

Amazon has the KomputerBay 2x8GB  1333Mhz Laptop RAM Upgrade for $59.99 with free shipping.  While we don’t have personal experience with this brand, it gets 4+ star rating from 500 Amazon reviewers, many of whom have put this upgrade inside their Macs.

Use OWC’s handy guide to make sure your Mac is compatible. For the most part, 2010 iMacs and most Macs  released in 2011 are compatible:

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