> Your only option there is to destroy a Monopoly or After Burner.

Not sure what you mean. krikzz sells a Master Everdrive if you want something suitable for multiple games. If you want a single game cartridge for a homebrew release, raphnet has you covered.

The one exception is if you have a Japanese Master System (or Mark III). These have a different cartridge connector that's shared with the SG-1000 and SC-3000 and flash carts are harder to find. You may be able to use a western SMS flash cart with a passive adapter though