Complete Mame Rom Collection
You used to be able to just download single roms from reliable websites whenever you wanted one, but since rom sites are getting shut down left and right it might be better to just get a full set to archive yourself locally. That's what I thought, so I went through Google, Reddit, Torrents and trying to find the newest possible versions of these romsets, since they still regularly get additions or corrections. 
Complete Mame Rom Collection Browser
To save you that trouble I wanted to share the links. Move whatsapp from ios to android. Info: No-Intro Collection (by ) Full collection of every clean dump of every real cartridge. Includes all different existing official versions of every game, but no romhacks, translations or homebrew. Download size: 37 GB Systems: The Everdrive Packs (by smokemonster) This pack focuses on sorting and being ready to use (with flashcards, but obviously works with emulators). It includes only 1 version of every game, the US version being preferred, with the addition of exclusives from other regions.