In order to download software you need a support contract which provides software upgrades support and the type of hardware which you are downloading for needs to be linked to that contract. In other words your AIR-CT3504-K9 needs to be on the support contract in order to download that software. That would typically be something like Smartnet Total Care. Best to discuss with your Cisco account manager or reseller.
But if you don't already have support on that 3504 you might have difficulty adding it now because it has already passed the End of New Service Attachment Date. (January 31, 2023)
https://www.cisco.com/c/en/us/products/collateral/wireless/3504-wireless-controller/eos-eol-notice-c51-744737.html
In that case you could find a recent security advisory and find the section which says "Customers without Contracts" then email TAC (don't phone) quoting the URL of the advisory, the paragraph just mentioned and the version and URL for the software you want to download and serial number of your WLC. Then TAC should publish it to you directly. This one should be suitable: Cisco Wireless LAN Controller AireOS Software FIPS Mode Denial of Service Vulnerability
"Customers Without Service Contracts
Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html
Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade."
