Metal Scrap can be a tricky resource to find if you don't know where to look. Metal Scrap, in its raw form, only spawns in contruction areas. But you can craft it from many different recources that are easy to find. Here is a list of everything you can craft into metal scrap as far as I know. If you know of something that isn't on this list mention it in the comments and I'll add it to the list.
Clothing:
A few different types of clothing can, instead of being turned into cloth, turn into metal scrap.
Construction Helmet = 1 Metal Sracp
Firefighter Helmet = 1 Metal Scrap
Desert Military Helmet = 2 Metal Scrap
Forest Military Helmet = 2 Metal Scrap
Desert Military Vest = 3 Metal Scrap
Forest Military Vest = 3 Metal Scrap
Police Vest = 3 Metal Scrap
Guns:
Every gun in the game (excluding craftable bows and the Matamorez) can be turned into varying amounts of metal scrap, from 1-7.
Colt = 1 Metal Scrap
Hawkhound = 1 Metal Scrap
Rifle = 1 Metal Scrap
Ace = 2 Metal Scrap
Cobra = 2 Metal Scrap
Masterkey = 2 Metal Scrap
Schofield = 2 Metal Scrap
Snayperskya = 2 Metal Scrap
Zubeknakov = 2 Metal Scrap
Bluntforce = 3 Metal Scrap
Compound Bow = 3 Metal Scrap
Desert Eagle = 3 Metal Scrap
Eaglefire = 3 Metal Scrap
Maplestrike = 3 Metal Scrap
Nykorev = 3 Metal Scrap
Sportshot = 3 Metal Scrap
Crossbow = 4 Metal Scrap
Timberwolf = 4 Metal Scrap
Dragonfang = 5 Metal Scrap
Honeybadger = 5 Metal Scrap
Rocket Launcher = 6 Metal Scrap
Shadowstalker = 6 Metal Scrap
Grizzly = 7 Metal Scrap
Vehicles:
When vehicles are destroyed they drop metal scrap. The amount of metal scrap dropped ranges from 1-8 and does not depend on the vehicle destroyed.
Miscellaneous:
Any other ways of optaining Metal Scrap that aren't listed above.
Metal Sheet = 3 Metal Scrap
Crafting with Metal Scrap
Metal scrap is used in many crafting recipies which is why it is so good to have. Usually metal scrap is used with Metal Sheets and/or Blowtorches to craft. Here is a list of everything you can craft with metal scrap and everything else metal as well. Note: Some of these items may require you to level up your crafting level. You can do this by going into the Support section of the skill tree and clicking Crafting once you have the required experience.
Tools:
Metal Scrap can be used to craft a few tools that, though aren't very good, can still be helpful in an emergency. These items can be found under the Tools tab in the crafting menu.
1 Can + 1 Explosive = Grenade
4 Nails + 1 Baseball Bat = Makeshift Bat
2 Can + 1 Tape = Makeshift Muffler
2 Metal Scrap + 4 Birch Planks + 1 Tape + 1 Rope = 1 Birch Rifle
2 Metal Scrap + 4 Maple Planks + 1 Tape + 1 Rope = 1 Maple Rifle
2 Metal Scrap + 4 Spruce Planks + 1 Tape + 1 Rope = 1 Spruce Rifle
6 Metal Scrap = 1 Rifle Clip
Supplies:
Metal Scrap is used for a couple of items used to help fight infection and fix broken bones. These items can be found under the Supplies tab in the crafting menu.
1 Maple Plank + 2 Metal Scrap = Splint
1 Chemicals + 1 Bricks + 1 Can = Vaccine
Gear:
Metal Scrap is frequently used for making other materials used for crafting. These Items can be found under the Gear tab in the crafting menu.
4 Metal Scrap = 1 Can
1 Metal Scrap + Blowtorch = 3 Metal Bars
3 Metal Scrap = 1 Metal Sheet
2 Metal Scrap = 1 Nails
6 Metal Scrap = 1 Wire