Don't think I'd ever say say 'ain't,' but I've only picked up the higher strata of speech patterns (closest thing to a mesolect I can speak is Estuary English) - at any rate, I'm fairly sure I've never heard 'ain't' in any basilectal speech either, and I have studied my share of accents and speech patterns. I'm fairly sure it's a US thing.
What is quite interesting is that many non-accent aware people trying to speak English here in Belgium will also say 'ain't', simply because of how prevalent it is in media.
What is quite interesting is that many non-accent aware people trying to speak English here in Belgium will also say 'ain't', simply because of how prevalent it is in media.