I worked with somebody that I knew fairly well beforehand. You have several options when it comes to finding a dentist to observe/asking to observe. First of all, try your general dentist. He ought to know you better than a random person on the street (we hope!).
I've observed 3 dentists over the last 2 years, for a total of 455 hours (yep, I kept track) That number includes the 4 hours I put in today too. I've found that generally dentists are more than happy to aid you in your quest with becoming more familiar with the profession. Be sure to be proactive in your observing. Initially this might mean asking appropriate questions, and getting to know the dentist better in general (everybody loves to talk about themselves!). As you develop a better relationship, you should find yourself becoming more active in the office, and not just sitting back or standing in a corner.
If you initially explain your goals to the dentist, they should be fine with it. My mentor is now completely comfortable with me second-guessing his choices. Although I NEVER do it in front of a patient, and I am NEVER correct, my dentist enjoys allowing me to come up with an "alternate" treatment plan for the patient. After I give my suggested plan, he'll critique me on why it wouldn't work, or why it would but isn't as feasible as the treatment he has chosen for the patient, etc.
Above all, be committed to observing/assisting. Don't just do it until your applications are in, or until you've reached a minimum number of hours. At those points it is okay to lesson your commitment to some extent (ex: once every three weeks instead of three times a week), but don't give the dentist the impression in any way that you weren't interested in him, his work, the patients, or the profession. All my apps are in and I'm still observing because I enjoy it. I've cut way back, but I still talk with the dentist regularly.
Whether you call the dentist or walk into the practice is probably going to depend on your personal preference. I believe you'll have more success if you do it in person, and probably the most success if you visit with the dentist around lunchtime, at the end of the day, or early in the morning before he starts his patients. Good luck, observing is a great experience if you do your part!