The A1286 model is very generic, applying to MacBook Pros all the way back to 2009. You need to specify exactly what model year you have. As the Maximum ram supported can and does change between model years.
You can use this Apple Page to identify your Mac more specifically:
How to identify MacBook Pro models - Apple Support
You can go to this page once you identify it, to find out how much RAM exactly it can take:
http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro
And no, no Apple Store, or authorized reseller will sell you RAM for your Mac, it does not work that way.
Suggested RAM sources are MacSales.com, and Crucial.com