how does a 'trainer-for-hire' work?
It's very freelance-y and varies from trainer to trainer, but I personally work with a lot of professors/archeologists/etc. Sciency stuff!
There's definitely a lot of networking involved. I managed to get an internship with Professor Rowan in college (after doing my gym challenge) and from there he just started sending me around to help with other professors and scientists.
I realized that, while I'm not super book-smart or studious, I love learning about science and history! So it's a great way to be involved without having a lot of technical knowledge. I'm good at battling and fieldwork, and a lot of my clients are too busy researching to train their pokemon enough for extra dangerous areas, so that's where I come in! It's a lot of fun.
I would say if you're interested in trying it out, contact your nearest ranger station and/or professor for info and suggested skills. There are quite a few of us running around!