Where lithium?

I only know of Voeld. You've got to get it in person.

There is a mod on the Nexus -if you are using PC version that gives you ability to get resources... I think.
My condolences if on one of the consoles.

You can get significant amounts of minerals from containers on any of the planets, but it's pretty random. On Havarl, the containers with minerals are actually labeled "mineral cache." On Eos, the containers in the two Research Center buildings have minerals in them rather than other stuff. How much you get and what minerals you get are pretty random though. There is also actually a fair bit of Lithium on Eos, but I can remember exactly where I ran into it and you have to collect it on foot... perhaps near the area where you meet Bain Massani. Voeld also have quite a few nodes of it that you collect on foot. The other source is, of course, your vendors and don't forget the buy/sell kiosk on Tempest.

Other sources are through Strike Team rewards, and activating the "mineral drop" AVP pod.  (Again, though, it's entirely random what you might get at any single time.)

The game is trolling me. I started an insanity PT from scratch, Level 1, and have clawed my way to level 24 with mostly found weapons and armor.

I managed to craft only one weapon at level 2 recently, because I have found only 2 units of Lithium so far, and need 150 or more to craft a piranha and a carnifex. As far as I can tell, lithium is a random drop, and I'm getting zero. Mineral containers have given me none.

I have almost no useful augments. I researched a couple, but I have found no useful augments in containers.

Most of my credits go to buy elemental ammo and stuff I need to craft armor. Containers have really been disappointing.

It's a tough game with squishy characters, weak guns and meh armor.

Exactly what I'm doing right now. Insanity, new PT, spending all my credits on elemental ammo.

Which why you need to hie yourself to Voeld and mine like a mad man.

Seriously, a Lithium thread and no one's made jokes? I'm almost disappointed...

I'm impressed, thread.

I guess I have to be the one to make a drug reference.

AT YOUR LOCAL PHARMACIST OR WITH A PRESCRIPTION DOHOHO

You can get loads of goodies running Apex missions frequently. Gear, resources, credits, and research - all available rewards from running them. Also, cryo pods. You don't have to play MP to receive the goodies from Apex - your teams will retrieve the stuff for you. There is a smartphone app available that will allow you to run Apex missions when you are not in-game.

The things your Apex teams retrieve can also carry over into a new game if you so desire. The character I was playing had plenty of stuff, so I let the Apex resources accumulate until I started a new character in a new game, and she was well-supplied from the very beginning.