They at least USED to have a daily flight back and forth en between PUP and PUJ which would stop in LRM. Here's their schedule page (however it does not seem to load fully today): Regular Flights
I don't seem to find a Sosua number (or any at the POP airport specifically) but here is their contact information from their web site: Contact
TakeOFF as well as other companies do offer charter flights in most all sizes.