The best way that I found is to make sure you look absolutely everywhere as Bethesda really likes hiding things in places you wouldn’t think of, as for a general guidance this is how I get most of my stuff.
Looting enemies – I think certain enemies have a higher chance of dropping things but not positive.
Inside containers – Search any and all containers, what item you get is completely random and will be different for everyone.
Completing quests – You usually get something for completing them.
Just found out in the world – Search everything you see including but not limited to – tables, shelves, lockers, workbenches, and boxes.
Hope this helps you!