Because "Plus" versions are fan-made hacks, they are often found on specialized ROM sites.

Elias parried. He rolled. He fought with everything he had, exploiting the broken mechanics of the Hack. He used glitches to cancel moves that shouldn't be canceled. He fought fire with fire, exploiting the game’s own broken code against itself.

on Steam, which includes the extra characters and balance tweaks without the "broken" nature of ROM hacks. SuperCombo Wiki

Elias stared. He hadn't even touched the ground.

: Put your kof98p.zip (do not unzip it) and your neogeo.zip BIOS into the ROMS folder of your emulator.