Uranus missions, specifically Ophelia or Stephano. Go for survival if you can. Bring a loot frame like Nekros or Hydroid. Makes a huge diff.
It drops in big chunks sometimes but still kinda rare. Patience is key.
Pair it with a resource booster and you’ll stack up fast. I do it during alerts.
You need it for some Railjack and Warframe stuff. Weird but necessary.
Only drops in specific tilesets so don’t wander off. Stick to Grineer underwater maps.
It’s grindy but not the worst farm in the game.