There are many used sail suppliers, such as Bacon in Annapolis or Minnie's here. Works best if near enough so you can try the sail on your boa.
You can get the measurements from sailboatdata. com. They're close enough to get quotes for new sails from sailmakers.
A local sailmaker, once chosen, will come to the boat and measure. It's a big part of their service, and puts them on the hook for getting the fit right.
Budget sailmakers like Lee and FX are worth exploring too, if not racing, and if you choose them you measure the boat yourself.
Prices vary wildly. What's the difference? Most sails are made overseas nowadays--it's the patterns and execution and service that make for the differential.
Local is persuasive, because they fix mistakes and miscues, and it's handy to have a pesonal relationship with an expert you can call for issues and advice.