My suggestion would be to do a Google Search (or Google Image Search) for 'balloon decorations car dealerships' or 'balloon decorations auto dealerships'. I found this website which features nice images of possible balloon decorations for car dealerships.
Also, keep your eyes open when driving round your area. Perhaps you'll spot some live examples for balloon decor in auto dealerships.
I haven't done this type of decor myself, so can't really give any personal recommendations.
