Titanium is a rare crafting resource in The Division 2 that is used to make guns, gear, and mods. If you plan on crafting a lot of items, then you will need a lot of resources to do so, and Titanium is a vital resource indeed.
Where To Find Titanium
The place to get the most Titanium is in the Dark Zone. Enemies in this dangerous part of the game are laden down with Titanium and can drop multiple pieces when they die. If you can make it to the Dark Zone, head for large groups of enemies and wipe them out, and this should net you some Titanium.
For players who cannot get into the Dark Zone yet, you will need to track down the Outcasts. The Outcasts are one of the enemy factions in The Division 2, and the specific resource they can drop is Titanium. Hyenas will drop Electronics, and the True Sons will drop Carbon Fiber.
Simply head to Outcast controlled areas and fight as many of those enemies as you can find, and some of them will drop a piece of Titanium for you. You can find Outcasts in the West End, Foggy Bottom, and Constitution Hall.
One final note, the various factions will drop these resources in missions, not just in the open world play. As such, you should come across plenty of this rare resource over time.