Historically the gold standard for MAME sets, now largely found through community-maintained mirrors and trackers.
Downloading complete ROM collections (often 50+ GB) encourages: Full Set Mame Roms Download
If you own original arcade PCBs, you can legally dump the ROM chips for personal backup and use with MAME. This requires specialized hardware (like a ROM reader) and technical knowledge. Historically the gold standard for MAME sets, now
When downloading ROMs, consider the following: Full Set Mame Roms Download
Some developers curate smaller "Lite" sets or "Best Of" collections if you don't want to download hundreds of gigabytes. How to Choose the Right Version
MAME ROMs are structured differently than other emulators due to "parents" (original games) and "clones" (variants).