Going on advice from InkBlend I converted the logo to a vector graphic from a sticker sheet scanned in for the 7945 Fire Station set on BrickLink.
I used VectorMagic to convert the file and automaticaly create paths. The SVG vector file can be downloaded here. If you don't have an image editor that supports vector editing, you can also download a high-resolution PNG graphic (267.33 kB).
You could also try scanning in a sticker sheet from a set that has the sticker. With Illustrator, Photoshop or any other similar image editors, trace around the design using a pen tool and fill in the colours. Using the scanned image as a reference.
If you're having trouble finding a suitable image, I would also recommend looking at the LEGO City product page for wallpaper downloads and illustrations.