This is an age-old problem.
Your purchasing department needs to be "advised" to include all required technical information in the initial purchase order. In other words, it should be one of the purchase conditions:- "No manuals - no sale"!
Another, albeit "robust", approach is to refuse to accept equipment without manuals (but be prepared to take some flak from the medical folk who "must have" that equipment).
It can be very difficult to obtain manuals once (after) the equipment has been bought and paid for. In the scenario you outline, you could try talking with the local agent along the lines of a promise of a service contract (2nd line) if they release the manuals so you can maintain at 1st line.
But in short, you always have to retain some form of leverage in situations like this.
