Lithium is pretty much everywhere. Trace amounts can be found in all life on Earth.
Lithium is a highly reactive element and so it isn't found pure in nature. You have to extract it from other elements if you want a pure form.
Just because it is consistently everywhere though, doesn't mean it's common. It is all over the place, but it isn't found in large amounts. That is probably a good thing because it is very corrosive. It even needs to be handled with gloves to not burn skin.
Lithium may be more help than just for mental disorder. It is still being researched for other possible medicinal uses like headaches. There are some issues that may arise from using lithium, so much more research is needed.