This time round, I made the mistake of telling Yes Man to ignore the Omertas, and consequently I didn't get the quest. I've tried talking to the receptionist, and talking to Mick about the guns, but there's no way to begin it as far as I can tell. I'm going Wild Card, so I can't get it from any of the factions.

Try pickpocketing Cachino?

He won't spawn until the quest is started.

You can get the quest if you complete Bye-Bye Love.