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.
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Has any one got any tips please. I have inflated my 24inch deco bubble, I put my vinyl on to personalise, but when I peel my transfer tape off, it marks