Well, I pay for Going (formerly Scott's Cheap Flights).

It's a pay site, but they constantly scour the airline sites for cheap fares and "mistake fares." It was originally designed for overseas/foreign flights, but the strategy changed during COVID.

The model is this: You specify up to five (I think it's five) departure airports, and they email you cheap fares that they find for those airports. For example, on Dec 28, they sent me an email for a trip from a nearby airport to SXM. $297 in January and February 2024.

I consider it money well spent.