For the Android user, you want or Split sets. Here are the standard community sources:
Places like or RetroGames offer curated packs with high-speed downloads. These require registration but are safer from viruses. Pack Roms Para Retroarch Android
Troubleshooting: If the scanner misses games, your ROMs might be bad dumps. Use the "Manual Scan" option and set the correct core. For the Android user, you want or Split sets