A common method is downloading a ZIP file containing the necessary models from video descriptions, such as the one found on this YouTube guide Community Repositories:

The HT12E (Encoder) and HT12D (Decoder) are essential components for simulating RF remote control circuits in Proteus. Since these are not included in the standard Proteus library by default, you must download and import them manually. 📥 Download HT12E & HT12D Library