Where do I get Golden Razz Berries from?

You can only get Golden Razz Berries as a drop from defeating a raid boss. The higher tier the raid is, the higher number of drops you can get, including Golden Razz Berries. I've personally received 10+ from defeating tier 3-4 raids.

From Pokemon GO Hub:

Pokémon GO Golden Razz Berry is a special variant of the commonly known Razz Berry. It can be used for two purposes: fully restoring a Gym Defender’s motivation meter and drastically increasing your chance to capture a wild Pokémon.

Golden Razz Berries drop in Raid Battles, along with Technical Machines and Rare Candy.

As stated on Niantic's website, Golden Razz Berry can only be acquired by participating in raid battles

We’re releasing four new items that can only be unlocked by participating in Raids: Golden Razz Berry, Fast TM, Charged TM, and Rare Candy.