Important legal note: ROMsets are protected by copyright. You legally must own the original arcade PCB for every ROM you download. Abandonware is a myth.
That said, the set is widely preserved in the emulation community. Search engines will find MAME 0.78 ROM set archive.org or fbneo 0.78 romset. mame 078 plus romset
The full set is approximately 15–23 GB (uncompressed), compressed about 12–18 GB depending on inclusion of clones and devices. Important legal note: ROMsets are protected by copyright
Managing a 0.78 set without tools is impossible. You need ClrMamePro or RomVault. Managing a 0
mame078plus/
├─ mamepp.exe (or libretro core)
├─ roms/
│ ├─ sf2.zip
│ ├─ mslug.zip
│ ├─ pacman.zip
│ └─ ...
├─ samples/
├─ cfg/
├─ nvram/
├─ cheat.dat
└─ artwork/
Do not download random EXE files from pop-up ads. For Windows, the best build for this specific set is MAMEUI 0.78 or MAME Plus! 0.78.