I found out that loot can be found in random encounters with caves. However, most of cool things you can find in the invaded caves (just watch out for deathclaws!).
So far I found Red Ryder LE BB, Sniper Rifle, Solar Scorcher (yes!), Enclave Helmet, 2 x NCR Ranger Combat Armour, Plated Boxing Gloves, Some Medicines...
You can try 31,32:34 near Junktown special encounter (Vipers vs Merchant Party vs Junktown Scouts) - mine's favourite. You have to search all the cave carefully because stuff is placed in really strange spots that are not visible from the entrance point of view.