You can still buy unused sealed OEM copies of Windows XP Professional on secondary markets (eBay, etc.). Prices range from $50–$200. These activate via Microsoft’s phone system.
Since Microsoft decommissioned most XP online activation servers in 2014, standard methods often fail. Here are the reliable alternatives: 1. Automated Phone Activation (Most Reliable) kms activator windows xp professional updated
Finding a "KMS Activator" for is actually a bit of a mismatch in terms of technology. KMS (Key Management Service) was introduced by Microsoft starting with Windows Vista and Windows Server 2008. Windows XP uses an older activation system that doesn't natively support KMS. You can still buy unused sealed OEM copies