Moss doesn't grow from seeds but from spores, so you might be hard
pressed to get moss spores.

The best method I have is to gather a bit from the outdoors and in this
regard the best ones are the type that grow in hot sunny and dry areas.
Yes it seems odd to find moss like that but look around.

I lived next to a field that was pretty water logged in the spring but
would have these tough mosses year round. in summer this field was very
dry. They seem to be able to withstand heat and drying out. I think
these mosses go dormant under extreme conditions. They can get crunchy
and dry but return again when the rains come. In a bonsai dish with
regular water they stay nice and green and spread.

I kept a stash of these in a section of my garden and harvested for the
bonsai as need.