MAVS is available on Github:
https://github.com/Mississippi-State-University-OTM/MAVS
To use MAVS with ROS-2, get the mavs-ros2 package:
https://github.com/CGoodin/mavs-ros2
For autonomy software that works natively with MAVS, check out the NATURE stack:
https://github.com/CGoodin/nature-stack
More examples that use the MAVS python interface for a variety of simulation tasks.
https://github.com/CGoodin/MAVS-Examples