For civil work, there are IME companies, though some are better options than others. You'll want to make sure to get your full fee, the cases are in line with your expertise, and reasonable terms (turn around time for deadlines, who does what, how long do they take to pay).
You'll want to find a mentor to supervise if you don't have experience doing IMEs already. They aren't something you want to "dabble" in because of people like me, Wis, PsyDr, etc...we love the casuals bc we tear amateurish reports apart. There are all sorts of different options with IME work: document review only, review + interview, review + interview + testing (depending on the case) and more. Some will have specific questions (i.e. interrogatories) to address, others will just expect you to opine about the treatments, diagnoses, recommendations, etc. Some will specifically want to know about if one or more diagnoses apply to the case.
Be VERY WARY of any eval or request related to child custody work. It's big $$, but those are commonly nasty cases and much more adversarial and ugly than most other civil work. I've had some "backdoor" attempts to get a psych eval and only during the interview do they disclose it's related to a child custody case. I have a disclaimer in my pre-paperwork that talks about child custody and other cases that I don't see, as I'll end the eval and charge full fee and not feel bad about it.
As for criminal, I'll punt that to others because that isn't my jam.
