I have complete BCGs as spares for each rifle, plus additional complete bolts. For my primary, I have a complete bolt and spare firing pin in the grip. As part of my go items, there is a spare complete BCG/CH and spare springs/pins/detents.
You can get bolt repair kits from PSA and such. Anderson sells oops kits with springs and such. At Academy, I bought a spring kit from Anderson and pins and detent kit from Black Rain, they were on the rack. Go to Primary Arms, PSA, Aim Surplus, Brownells, or any of the ads at the top or bottom and search for repair kits, AR parts, you will get hits. If you go to a gun show, you can get lot of spare parts too.
Basically, every item to make a new rifle I keep as a spare, plus spare rifles. Don't forget spare batteries for optics and flashlights, get lithium batteries for those items, keep spares of those in your go-bag too!