@SarahGen-- As far as I know, some research is done about experts in that vocation and they are contacted and requested to be expert witnesses. Some experts are actually affiliated with certain companies who assist attorneys and courts who need expert witness testimony for a case. So I don't think it's a difficult process. Of course the expert has to submit some documents to court such as their resume, experience, educational background and research. The court has to accept the individual as an expert and approve of his or her ability to testify on the matter.
Some experts take up duty as an expert witness on request while others actually do this as a full time job. This is why fees vary among vocational expert witnesses. The court or attorney and the expert decide on compensation together. It depends on the individual's level of expertise and how much work is required of him or her for that case.