Online there are many, like thesailwarehouse.com, but also I would try local sail lofts in your area, they may have or know who has a sail that matches your boat.
My experience with the local guys here is that, yes, they would like to sell you a new sail, but they'll also go out of their way to help you in any way they can - presumably so when you are in the market for a new sail you'll remember them fondly.