Since it seems that you are just starting out, I'll assume that you want to keep your budget low and specialized. Here are two low-cost methods:

1 - Face-to-face. Make phone appointments with the offices in your market region. Meet with the professionals in question and introduce yourself and your service.

2 - Direct mail. Send out brochures to the above-mentioned offices. Follow up a 3-5 days later with a phone call to answer any questions. Ensure that your contact information is clearly noted on any communications.

Hope this helps!