"Where to buy" questions are better answered if you give us some information about where you live. These are international forums and it does not help you if we recommend a vendor who cannot ship to your location.
I buy all our Mac RAM from one vendor, Other World Computing. They do some overseas shipping but you will need to check their web site to see if your location is included. This is their link for your MacBook Pro:
MacBook Pro RAM Upgrade Kits (2012 - 2015) I Warranty Included
I have my Mid 2012 13-inch MacBook Pro upgraded to 8GB using memory I bought from them and if has worked perfectly.
There is no "official" or "genuine" Mac RAM. Apple has used different brands over time.
RAM from Crucial.com is also first-rate. Like OWC, they have a presence in some countries outside the USA.
Some RAM brands have proved troublesome. Both Kingston and Corsair have two levels of RAM. The higher-priced upper level product seems to work OK in modern Macs. It is the lower-level "value RAM" that can prove troublesome in Macs. The probelm is the RAM market is highly price-sensitive so many vendors stock only the lower-priced lines that don't work well in Macs. To get teh better grade you usually have to order directly from the manufacturer.
Also avoid PNY. I used it all the time before the advent of Intel-powered Macs in 2006; today it is the subject of too many problem reports in these forums to make me comfortable in recommending it.