I agree the Yellow Pages are very expensive.
Are you able to go to conventions, workshops, seminars and exhibit your business? I do that with my Medical-Legal consulting business.
Have you considered creating brochures and/or newsletters and sending to current/potential clients, outlining what your business has to offer?
Are you a member of your local Chamber of Commerce? You can normally get great deals on advertising with what they send out in the County.
If you have already established contact with your client(s), you could utilize email updates/interesting information regarding your line of business.
You could try sending "discount" flyer services to current/potential clients.
Have you tried luncheons for clients? Get in the door and do a presentation of your business.
We do not have the same business, but are Entrepreneurs and can share similar ideas.