I've been lettering up a couple of race cars this week. I used to go to a local sign shop, but the costs drove me away. I have to admit that an inexpensive Roland vinyl cutter made all the difference for us. I justified it for other business reasons, but I'll bet it's pretty much paid for itself in race car stickers alone.
We cut the stickers showing our sponsors ourselves, because that way we can make it just the right size and integrate them well into the car liveries. The end result is a better looking car, which benefits our sponsors. For example, the car has V8 Roadsters across the front air dam. It's curved, of course, so we had to get just the right arc and dimensions. There's no way V8Roadsters could have provided that for us without a bunch of back and forth. Same with other sponsors - and when I made a mistake in spelling a name, I was able to quickly correct it on the car without having to pay for another run of vinyl from a local shop. If you can do that sort of work, you'll probably pick up some attention.
The cutter is not an option for most people, of course. To answer the original question, I'd start by contacting the local car clubs and let them know about your services. Maybe offer to do a run of club stickers for them, which will build you a bunch of good will and get you some attention. There's a big advantage to being local - you can turn around decals a lot faster than an online place. And we all know how racers plan ahead 
The idea of doing a package for a local high-visibility racer and making your name part of it is a popular one, I've seen a lot of sign shop names on race cars. So I'm guessing it works.
