my buddy bought a vinyl cutter machine and we did our own. We even added shark graphic and bubbles. It was great!
There is a bit of an art to picking the vinyl and application if you have multiple layers, but ours still came out great. Make sure you ask not just about the vinyl colors, but the quality too. They sell the stuff in 5yr, 7yr, etc.. quality levels and you really want something better than the bottom of the line.
Just about any sign shop can do this, but you are better off finding somebody that can really work with you on the design. You really can do just about any font and coming up with a picture of the transom (with measurements) can be a great way to figure out what size of graphic you really need. It's really really hard to just look at a book and decide the right size to print with. You really need to have the context of the size of the boat itself.
If you put it on yourself, work out a plan with the person doing the cutting. The cost of this is all labor. The material cost is very very small. If you screw something up, don't fight it. Just go back and have the printer cut a new piece. Don't be afraid to ask them as it won't cost them much at all once the file has been saved.