Several options are available for finding a consultant. For starters, ask friends, family and colleagues. Getting referrals is always a good way to start because someone else has done much of the legwork and you get the benefit of what they've learned. This is the easiest and quickest way to find a consultant.
The Yellow Pages, surprisingly, is also a good source. The "Consultant" category is broken down by type (such as management consultants and business consultants), so you should be able to find one that matches your needs. (But you'll still need to do extensive research, which we'll get to in a moment.)
Trade journals are also helpful. Consultants tend to advertise in journals the majority of their potential clients read. A word of caution, however: An ad placed in a journal is not an endorsement by the association. Generally speaking, anyone can advertise if they pay the fee for the space. Therefore, you have to do your homework.
If the invention you're working on falls into a specific industry, join a trade association in that industry. This can be a great way to get in contact with consultants--and a great networking opportunity for finding others who can help as well.
A fairly recent phenomenon in the consulting field is companies that act as a clearinghouse to match your consulting needs with the proper consultant. The Consultants Bureau (http://www.consultantsbureau.com), created by the International Trade Association for the Consulting Profession is one such resource. Once at the site, you simply fill out a worksheet in which you pick your requirements from more than 200 consulting disciplines (such as start-up businesses, business plans and pricing), your need (such as an immediate project or a writing assignment) and your industry. The association then polls its members and gives you appropriate names and contact information--all free of charge. Keep in mind, these services typically only offer a list of members who fit your needs and don't recommend one member over another.
Another site, http://www.referrals.com, created by National Consultant Referrals Inc. (NCRI), operates on a similar concept. NCRI works with you to determine what kind of consulting you need. This can be helpful if you can't put your needs into a preordained box. After speaking with you, an interviewer matches your needs with suitable consultants in the organization's membership of more than 6,000. NCRI does a thorough reference check on each consultant before accepting them for membership. Consultants must also provide evidence of expertise in their given field.
Another interesting resource for consultants is FIND/SVP, a research and consulting company based in New York City. The company has more than 100 in-house consultants and over 1,100 consultants worldwide who do longer-term consulting. It also has a service called "Quick Consulting" (call 800-346-3787 for more information), where it will arrange a phone call between you and a consultant with expertise in your field of interest, billing in six-minute increments.
FIND/SVP is primarily a membership-based service. The monthly fee can range from $500 to $10,000 per month based on your industry and the number of users. However, they will take on ad hoc projects for fees that start at $500 and will provide you with a free proposal prior to requiring a financial commitment. To contact FIND/SVP, visit http://www.findsvp.com or call (212) 645-4500.