Because the product is end-of-life, Cisco has removed the ISO from its public software download center. However, it may still be accessible under very specific conditions:
Because ACS is obsolete, Cisco recommends migrating to ISE. ACS 5.5 is a critical "stepping stone" version for these migrations. Cisco Community ACS GUI password reset - Cisco Community
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: While you might find trial keys online, they are useless without the base ISO image, which Cisco no longer hosts publicly. Cisco Community Recommended Migration Path Cisco has replaced ACS with Cisco Identity Services Engine (ISE)