Peter Johnson are about $3 a letter and would have to most probably make them to order as the height must be minimum 230mm, width 150mm with 30mm line thickness. (you may be lucky and they have them in stock, sounds like Russ was)
In your case FOS 3 letters and 4 numbers = 7x3= $21
and you need to have both sides marked. 2x21=$42 for one sail to be marked.
The good thing about spending the money is that the numbers should last for the life of the sail and they will be there for any event you go in.
The bad thing about spending the money, is spending the money if you only go in one or two comps.
p.s. don't forget you need a hyphen between your letters and numbers. PJ will throw that in for free.
www.windsurfing.org/sailnumber_rules.htm