First, see if the patent is still in effect. It may have expired. If it is still in effect, you have to determine who it's legal owner is. Was it left to you as a bequest? Martindale Hubbel (see your local library) has separate listings for patent attorneys (in the blue pages at the beginning.) Good luck!
I would first get recommendations from people you know who have employed attorneys. For instance, do you know somebody at work, at your church, synagogue, or mosque who has employed an attorney? Did they have a good experience with that attorney?
You could ask certain businesses for recommendations. For instance, if you are purchasing a house, perhaps your Realtor or title company could recommend an attorney.
If you need to have some estate planning work done, you might ask a bank trust department to recommend a good estate attorney.
Next, you might go to the Martindale-Hubble website, http://www.martindale.com. You can search for attorneys by name and by town. I like the Martindale website because it includes attorney "peer ratings", and that means a lot because it enables you to get one who is well regarded.
Of course, you could also open the Yellow Pages of your telephone book and just choose one that has an appealing advertisement.