Approach them in the slow season.
Best would be if you had a cutfile ready for the sailmaker. Then you would be buying time on his cutting machine and for cutting/packaging, not his intellectual property.
Sailmakers do this for a living. Either he see easy money in cutting and supplying materials or he see somebody out to take money away from him by not letting him spend time assembling and finishing. Might also be percieved as a PR/quality/hassle issue. All in the mindset of the sailmaker.
It is not wrong to ask
Why not go with Sailrite? I have built a few of their kits and they are OK.