You can go several different routes: used, you measure it new, or they measure for you new. Personally I am not that keen on used sails as they offer generally poor performance and an uncertain life. There are several places listed in the back of Cruising World that stock used sails. Both this website and places like www.cruisingdirect.com offer new sails constructed from your measurements. You get a new sail from a quality manufacturer. (North Sails makes Crusing Direct). You need to measure youself which is not a big deal. The high end in terms of cost would be going to a full service sailmaker like North, Quantum, etc. They would provide a person to go out and measure for you, advise as to cloth weight, reefs, etc. You would also expect excellent after sales service and even a test sail. I have had very good experiences with North Sails but I am not all that convinced that one maker is vastly different from another. I would be concerned with sailmakers who are physically far from your boat as you may not get the service that you need in case anything goes wrong. Good luck.