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
Unlike modern MAME, which re-dumps and renames ROMs with every update (breaking older sets), the 0.78 set is frozen in time. If you download a ROMset labeled "MAME 0.78," it will work perfectly with the 0.78 Plus emulator forever. Unlike modern MAME, which re-dumps and renames ROMs
The MAME 0.78 Plus ROM set is a specific, highly-regarded snapshot of arcade game ROMs corresponding to MAME version 0.78 (released around 2003–2004), often paired with the unofficial "MAME Plus!" derivative emulator.