Use a directory. The phone book is the most obvious, but doesn't tell you about services. You can check a more specific listing, such as the Consultants and Consulting Organizations Directory published by Gale Research Company of Detroit. The Better Business Bureau and other groups also publish directories in many areas. Check with your own professional organization.
Some consultants list themselves with professional organizations in the fields they serve.
Check with your accountant. Many accounting firms have separate computer consulting services. Those that don't can refer you. Check with the Better Business Bureau or the Chamber of Commerce in your area. The BBB collects complaints and can help you screen out consultants with bad reputations.