Look for "Internet Archive" (Archive.org) collections, which often host historical software preserves for educational purposes. Why Enthusiasts Still Love It
Historically, multicarts were produced by bootleg companies primarily based in Taiwan and Hong Kong for the Famicom (the Japanese version of the NES). These collections were later adapted for Western consoles.
The "300 in 1" NES ROM represents a classic era of "multicarts"—unlicensed compilations often found on bootleg cartridges or clone consoles