Interpreters regardless if they're with a school district, medical center network, or an agency...they are nearly always on the lookout for more opportunities. They are what is known as freelance interpreters, often they do not advertise but if you ask them for their business card, they'll give you one.
This may not be significant in large populated areas but in smaller populated areas where businesses and doctors office are smaller but employ more than 15 people per the ADA law. It is very important to keep those business cards because often we'll hear from the local business/doctor's office that they're unable to find an interpreter.
While that may be true but at what resources did they use? Give out copies or give them the contact infomation from the business cards over the phone or e-mail. This will greatly improve the chances of having an interpreter available. However, if there is none still available, I highly recommend calling the ADA's 800 number to file a complaint.
Some of you may not take this seriously because you can read and write well enough to exchange notes with your doctors or so. Please think of the others in your deaf and hard of hearing community whereas they cannot read or write well.
To wrap this up, here's a link to the Registry of Interpreters for the Deaf, Inc., (RID) searchable database to find interpreters or interpreting agency in your area.