What's your budget, where are you flying from, for how long and when? Have you tried plugging it into Expedia? That's how I usually start my comparison. When I find packages within my budget, I then go to TripAdvisor and read up on reviews.
Try looking on bookit.com and then cross-reference places on there with reviews on Trip Advisor. I've used bookit several times and really like them. They have really good customer service, and they'll allow you to book hotels and flights and pay in installments.
I booked our trip to DR through costcotravel.com, got a great deal and really enjoyed our time there! We stayed 7 nights at the Colonial Majestic, upgraded to a swim up suite on the adults only side of the resort with access to the club house.
We have several friends who've been to the Colonial Majestic, including 2 who eloped there. Compared to a Sandals it's a great deal. Be careful going in March, the weather in the DR is supposed to be awesome (not too humid, no hurricanes) etc. so it books up and rates are higher. I've heard the adults only section of the resort is worth spending a little more.
Also I believe Jet Blue goes direct to DR, however it's only certain days. You might want to run flights through Expedia to see if you can save more by leaving on an "off"day like a Wed or Thurs.
Re: Domincan Republic