I used to work for Continental Airlines several years ago.
The reason airlines overbook is because of the no-show rate - people will book and not show up.
So in very broad terms the country broke down like this
Midwest - 10%
East coast - 15%
West Coast - 20% no show and if I recall - this is over 10 years ago SFO flight consistantly were aroind 25% no show rates or more.
So look for flight that are heavily booked out of the midwest ---
In the West coast you could be over booked 30 pax on a 150 pax flt and still go out with open seats.

Also the airlines used to have to report to the FAA overbookings. I can't remember now if it's an overall number ie we denied xx pax per yy flights or if there was more detail.

Someone may have the power to poke around there and see.

I hope this helps.
Frank

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TANSTAAFL - but if you work it right, FF miles comes pretty close.