Juanita,
I frequently fly to SD or STI. You can sometimes find good charter deals thru POP or PUJ with limited dates but....
Almost always, the best prices to S or STI are through BTV (Burlington, Vermont). This airport is about 1 hour drive to MTL. It's well worth it for the savings. Everyone from MTL uses this airport for flights thru the US nowadays. Generally the connections to the DR from BTV are thru JKF, MIA or ATL
Québec is 2.5 hours from Montreal, so at that distance you might as well include Ottawa....
Check out this link for prices to BTV
http://matrix.itasoftware.com/cvg/di...597C8D02D11143
Good luck,
Montreal