There are a lot of locations in Fallout 4 that contain a large amount of junk. Its hard to say the best place to find junk, but If you are looking for large amounts of junk quickly the best place you can go is likely the institute. This place is loaded from top to bottom with loads of very rare items that yield a large amount of crafting components. If you can do a few runs up and back you will earn yourself a huge amount of crafting components to use on your workbench.
Items to look out for in particular would be microscopes which will can be scrapped to make a lot of good items. Most of these can be found in the science areas, locked away in cupboards. The living quartets will also contain a large amount of good crafting components.
The best time to do this is before you have triggered he end game, bunker hill quest. You don't want to have any fractions hostile towards you. Strip naked and leave all your guns and items at your settlement. Go to the institute and take all of the items you can get your hands on. Walk through all of the hallways grabbing every piece of junk you can set your eyes on. Some of the items can weigh a lot which is why I suggest you go naked with nothing else in your inventory.