Where does Apple recommend can I buy memory for my Mac?

Apple doesn't recommend any RAM vendor. They are impartial.

Try Crucial, http://crucial.com or OWC, http://eshop.macsales.com.

or

http://datamemorysystems.com

All of the above ascribe to this important rule:

"It works in your Mac,

or your money back!"

Nothing less will do.

NB> If outside the US, post back with your buying area and readers can make different suggestions.