matrix.itasoftware.com
--You have to input the 3 letter airport codes you're interested in (can't just be USA). But I don't think there's a maximum of possible departure cities.
--It only searches so you can't book from there. Have to then go book it at your choice of airline flight reservations sights.
--There's also a bunch of ways to drill down on what you may be looking for.
For example, here's the results of a search for a flight from one of several US airports to BKK, HAN and KUL flying Business class for a 13 day stay anytime during April https://matrix.itasoftware.com/calendar?search=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%3D%3D
(also - there are sites you can subscribe to that will alert you to special or mistake fares when those become available.)