That’s a great question unfortunately without a solid answer. I am all about the the importance of soil fungi, but I am not quite so sanguine about the mycorrhizal fungi additives.
It makes sense that you could find a nice source of the appropriate fungi, take a little sample and transplant it into deficient areas. But what I have been told (and I would love to be proven wrong about this) is that the specific species that are most desirable simply don’t grow well in a lab for easy harvest and distribution.
I am not saying that those little packets are bad, only that they may not give all the desired results.
But there is good news. These species still exist, often in ditches and shelter belts. I also understand that their spores still exist in soils just waiting for proper conditions to arrive. The proper conditions being lack of herbicide, pesticides, fungicide, etc. in addition to not using chemical fertilizers.
Basically, a lot of areas have a “build it and they will come” situation.
So I say use those packets and hopefully the remnants of the appropriate fungi will re-emerge.