I dont know of a website but lots of racing dinghys buy new sails every couple of seasons the old ones are fine for daysailing.,If theres a dinghy racing club near you try to put a wanted add on their noticeboard or website.
you could try a wanted add on on this forums wanted section.Go to the main index then scroll down.
Make friends with the dinghy section! I would have thought that somebody could help out. Certainly an advert on the notice board with luff length and mast height should do the trick. Seateach have been great for us in the past, but I'm not sure how small their sails go.