As the 3500 series is end-of-life, the software is generally archived. You can attempt to download it from the Cisco Software Download

This specific file is an image for Cisco Aironet access points.

: This specific version is often cited as the "ultimate" or final version for these legacy devices. Cisco Community Downloading the Software

: Ensure that the firmware or software version matches your device model and current software version to avoid compatibility issues or bricking the device.