Looking for an expert witness? Start by looking at nearby universities. In his book, The View from the First Chair, Martin L. Grayson shares this anecdote--
The last time I needed a toxicologist on a case I was handling, I called the Department of Chemistry at UCLA, the biggest university campus in my area. I spoke to a professor there, and after describing the issue, he didn't hesitate to refer me to a colleague across town at USC whose research focused on the exact chemicals in the exposure case I was defending.
It's good advice. Most professors are skilled at explaining difficult subjects to laypersons, which is exactly what you want in an expert witness. If you take the alternative route of relying on one of those ubiquitous expert mills, you run the risk of hiring an expert who can be easily branded by your opponent as being nothing but a hired gun.
Related post: "Book Recommendation: The View from the First Chair."