How to find the game

[BigHector]

I look up Mafia in the SC2 custom game section and nothing comes up.

[ypmagic]

Arcade -> Browse Games -> Look around the 6~8th rank.

[RLVG]

After you have found the game, bookmark it for easy access.
It's possible to search for games "Mafia", I believe it's called "-Mafia-".

Move this from General to Mafia Discussion?

[Raptorblaze]

It's probably because he's looking under Custom Games instead of Arcade.