I need to shoot several (~ 30-40) videos with fitness exercises in the gym.

I would hire a personal trainer. There are many semi-profesionals, who want to make money with fitness and it's very hard for them, so i can imagine, you will find someone, who will make it very cheap.

Order what?

Order a camera man or rent a gym?
If you need a gym maybe ask in your local crossfit box or another kind of gym depending of what kind of gym equipment needs to be there