I have used these guys from San Juan to both Beef Island and San Juan to Nevis over the years. Very reliable, and in my experience competitively priced. I call them a month or 2 before planned travel dates and they have never failed to deliver. I have also used Fly BVI in the past - and I have always gathered quotes from them, Island Birds and a few others. These guys have been my pick each time.

http://charterflightscaribbean.com