To make life easy here is a guide of all of the valuable items for crafting In Fallout 4.
With the workbench DLC on the way, this seems like a perfect time to start hoarding some junk so you aren’t left having to scavenge around the wasteland for junk that you could have found in advance. There is nothing worse than finally unlocking a perk to build that weapon mod and discovering that you are missing something like fiberglass and be left wondering, “Where the hell am I supposed to find that?”. There are a few crafting items in particular that are quite difficult to find.
When crafting you need to scavenge as much junk as you can from across the wasteland.