You can buy this fungus to grow in your flower garden if you wish.
If you read up on the way this particular mushroom is ingested, you'd soon discover that the active ingredient is NOT metabolized in the human (or reindeer) systems, but is excreted by the kidneys into the urine.
You would have also discovered that upon initial ingestion there is yet another substance in this mushroom that gives you stomach cramps and other uncomfortable symptoms.
Accordingly, you find a shaman or witch-doctor to eat the mushroom. Only he need suffer the cramps. Then you collect his urine and pass the bottle. Supposedly it is possible to run this material through the kidneys of 5 individuals before the concentration is too thin to give the desired effect.
Better yet is to feed your reindeer vast quantities of amanita muscaria. They love the stuff. Some sources say that reindeer are so addicted to this mushroom that should a Saami unloosen his trousers to urinate, reindeer will come running! (No doubt that's an apocryphal story about how to catch a wild reindeer). On the other hand, National Geographic has a film of reindeer eating absolutely huge quantities of this mushroom with tremendous gusto. Later they are shown urinating into buckets!
Add 2 and 2 together and you can see that the water in frozen urine is going to end up sublimating away leaving behind a yellow or brown powder just choc full of the active ingredients!
It is not to be believed that the Saami didn't happen upon such a process, particularly since this particular mushroom has it's peak growth period sometime in August. Whatever would the shaman do in the other 11 months if they didn't have a year round source?
In Ghengis Khan's day the Saami may well have had the best stash available, and they could produce it in large volume. There were no secrets from the Mongols.