Microsoft provides for Surface RT and Surface 2 (both run Windows RT). These are not ISOs—they are compressed recovery image ( .img or .zip ) files.
A quick search for "Windows 8.1 RT ISO download" might yield results on "abandonware" sites, torrent trackers, or obscure forums. download windows 8.1 rt iso
For 99% of users, the best course of action is to accept that Windows RT is a discontinued, unsupported platform. Recycle the device properly and consider a modern ARM device like a Surface Pro X (which runs full Windows 11 on ARM, not RT). If you must preserve old software or files, make a backup and store the device offline. Microsoft provides for Surface RT and Surface 2