RetroArch Alt is an alternative Front-End interface to RetroArch... (read on if that really didn't make any sense whatsoever).
Distributing them is mostly illegal, so again you won't find them here.
You can freely download the cores though. The cores are plugins which are used by RetroArch to emulate a specific system. The great thing about this is any improvements to RetroArch compatibility can be upgraded simply by swapping out an updated core file.
Lastly, BIOS files are sometimes required for greater compatibility. Again, the same situation as with the ROMS; you won't find them here due to copyright issues, although it's possible to play many systems without the BIOS files being present.
Lastly (and this will probably only make sense when you come to use the program), RetroArch Alt will pick the relevant emulation core for you based on the ROM it thinks you are opening. You can modify which core is used manually with the drop-down lists, or you can edit the file 'matching_rules.txt' to have RetroArch Alt always pick a specific core for you.
If it's easier, here's a Youtube video of how to setup RetroArch Alt.
RetroArch Alt is free.
version 1.0 - December 2018: Initial release
version 1.1 - January 2019: A few small tweaks to the interface
Tested with Mac OS X 10.7 up to MacOS 10.14 Mojave.
If you have any suggestions, please get in touch via email. Many thanks.