Happy customers will be more than willing to give you a testimonial to use on your site, and on external directories like Google Places.
On your site, you'll have to add their testimonial yourself, so just ask them to send it via email. But when they do that, ask them if they would also be willing to write the testominial as a review on Google Places, or if they would be willing to "tag" your business on Facebook or another social media platform to put in a good word across their social network.
Remember, the internet is the new word-of-mouth, and testimonials can go a long way. Good luck!