If you own a physical ASA 5500-X series device, you can legally extract the software image from the device's flash memory for lab use. Setting Up ASA 9.1 in GNS3
You can purchase a subscription to CML (formerly VIRL), which provides access to virtual images like ASAv that are highly stable in GNS3. gns3 asa 9.1 image download
After booting, you may need a specific activation key to enable features like "Multiple Contexts" or "AnyConnect". Important Stability Note Cisco ASA - GNS3 If you own a physical ASA 5500-X series
Once you have the files, follow these steps to integrate them: Important Stability Note Cisco ASA - GNS3 Once
An empty disk image, such as empty8G.qcow2 , is often used for flash storage. How to Get the Image Legally
Log in to Cisco.com with a CCO account that has an active contract to download the binary files for your specific hardware.