TL: H has already listed all the charter cos w/locations. Another options some choose is Horizon out of Grenada but with a Union Island departure which saves sailing time from their main base.
Here's a recent trip report (well done) that has some details:
Traveltalkonline: Sailing in the Grenadines - Long trip report
Also a favorite web site with real good info still help even after a few years:
The Usual Suspects - Destination: The Grenadines
Definitely Pick up the Doyles Cruising Guide for Windwards.
When are you planning to go?