You can purchase STORM for Microsoft Flight Simulator through a number of outlets. Below are the official outlets where STORM is available from:
STORM will also be made available through the in-sim Marketplace for both PC and Xbox users. This will follow a few weeks after the initial release on May 31st 2022.
OrbxDirect - https://orbxdirect.com/product/sofly-storm
FlightSim.to - https://flightsim.to/product/storm-for-msfs
FlightSim.com - https://store.flightsim.com/product/sofly-storm-for-msfs/