I ordered mine from the local Hobie dealer. They had a book with lots of different fonts and sizes. I chose a font and size that I thought looked good with the Hobie logo on the sail.
Applying them straight is a little tricky, since the cut of the sail is varying angles. I laid my sail flat on the floor, and used the Hobie logo as a "straight" reference. I used a t-square to draw my guide lines, but you could just lay a big box below the bottom of the logo, and draw a pencil line on the sail along the opposite side of the box. When you put the numbers on the other side, you'll want them offset so they can be read when the sun shines through; so maybe use two boxes for that guide line.
Lay your numbers along the guide lines and tape them down where they look right, then remove the backing one at a time to apply permanently.
What - Me Worry?
2006 Hobie Wave