First, I have to disclose that I am pro-LEO. These comments are for any experts and assumes you haven't used an expert in a particular field in the past.
Start by defining the exact area of expertise. You've mentioned ballistics and firearms. Very often, these are different people. The ballistics expert may be able to testify about the rifling on a bullet or the machining marks left on a cartridge. A firearms expert may be different in that they might be able to testify that it was possible for a particular gun to discharge because of a fall of three feet onto the ground. Sometimes, the same person may be able to testify as an expert in both firearms and ballistics, depending upon the depth of analysis need. A forensic pathologist might be necessary to testify about the trajectory of the bullet and how it was possible for a single bullet to cause three different exterior wounds. Government employees who retire may be a good source for expertise. For example, the former chief medical examiner in this state has a lot of name recognition and now testifies as an expert witness in both criminal and civil cases.
Unless you can get THE expert in an area of study, you're generally better off with someone local or within the region. Otherwise, it may appear to a jury you're buying testimony. At times, you may have no choice but to go outside your locality or region. Sometimes, calling another attorney will be the answer. Sometimes, calling the local university may uncover somebody or they may be able to refer you.
When all this failed, I used to use a service called TASA which is at
http://www.tasanet.com/. There are other services out there as well.
I know all this is pretty general. Spending a couple of hours locating an expert witness in an off-beat field is not that bad.
