I think that you need to specify where you are. I am writing from Melbourne, Australia, but you may be in Dublin, Winnipeg, or Kuala Lumpur. This is one of the problems with and benefits of internet-based forums; they're international. We have a local company who advertise secondhand sails for sale, with all the measurements and an assessment of the condition. However, this may not be of much use to you.
Peter.
I imagine it's for a robert Clark Mystery, and he's based in Lincolnshire. There aren't many of them around Mulligan, but you could always write to our letters and contacts pages. Specify the dimensions if you're prepared to take other sails not designed for the boat.
Dan