Any vinyl/sign shop can make graphics for a mylar main. The dacron fabrics sometimes are tricky to apply but mylar receives vinyl graphics very well. In addition to my day job, I own a graphics/sublimation business and we do our own team sails for Team Seacats (we don't normally do big graphics for other teams because our equipment and the resulting graphics are on the small side resulting in longer installation by having to panel large graphics).

So...go to anybody - Fast Signs, etc. I know Robi from this forum isn't too far away from you and he's done a lot of boat work.

Keep in mind that the more information and the better the file you hand to them will result in lower cost and a better result. Also keep in mind that if you pay the extra for a high quality (5+ year (9 year recommended) rating) and it will remove more cleanly from the sail should you need to remove it in the future. Cheaper vinyls will hold up well but will leave a lot of residue when you remove them that will be nearly impossible to clean off.