Search for the exact phrase "MAME 2003-Plus Reference Set" . Digital preservationists host complete, verified non-merged sets there for public download.
Here is the reference article and the necessary information regarding for MAME 2003 Plus. mame 2003plus reference link full nonmerged romsets
Games built on shared arcade systems (like Neo Geo or Capcom Play System) require specific BIOS files. In a non-merged set, these are usually built directly into the game zip file, or kept as a standalone zip (e.g., neogeo.zip ) in the same directory. Search for the exact phrase "MAME 2003-Plus Reference Set"