Go to your local aquarium and ask them to squeeze one of their filter media into your bucket, bottle, bag etc. Pour that into the new tank.
I have a current 10gallon going with a Eheim 2213 (and an older filter that provided too much flow but has been used for ages and I will be using) and tank gravel. I bought a second 10 gallon as I knew there would be the cycle in time that was unavoidable with all aquasoils and this seemed the easiest, just transfer everything over once the ammonia was done leaching.
My plan was to do a 30-50% water change from my existing 10 gallon with a gravel vac and have the water go into the aquasoil amazonia 10 gallon in order to get as much preexisting bacteria as possible then fill the rest of the tank with regular tap water. I would then run the filter 24/7 for the next 4 weeks, doing water changes/adding plants/etc. Any suggestions as to how to get the initial source water for the quickest cycle?