I spoke to several of our Reps for that work and various high end gun manufacturing companies and they told us that they are no longer taking orders for upper receivers, only complete firearms.  We pretty much figured that out when we saw the panic buying and the surplus of gun parts just disappeared.  I was involved with a company called Section 8 Tactical several years ago and we sold an M4 parts container kit that I would love to revive again someday.  I would hold a bolt, extractor and all of the parts you need if something on your bolt carrier group goes down.

If you snoozed for the last 4yrs and didn’t get a spare parts kit, you might have issues for several more months and possible years trying to acquire them.   Trying to find AR15 Complete upper receivers in a climate like this is going to be difficult but keep your eyes open for reasonable deals.   Prices will eventually come down to some degree but trying to get the good stuff is going to be difficult.  Some of the higher end products tend to not be made in high volumes so those waits are probable close to 1yr if you are lucky.