If you are working on a project aimed to this type of difficult to reach user and you still want to keep a proper Lean UX approach, how do you face the problem of finding users for gathering insights or testing prototypes?
I had to do a test with lawyers once. Finding this type of participants can be challenging and many recruiters flat out refuse to do it.
Your best bet is probably to use a list of existing customers and simply call those in the area you want to run your study. I have no hard evidence to back that up but I suspect incentives do not make much of a difference for this type of participants. However, if your product is important to their job, they probably appreciate the opportunity to give feedback (even or perhaps especially if they are frustrated with it). Of course, one drawback is that it won't tell you how easy or learnable your product is for new users and the test participants might compare the new design with their current experience and err on the side of conservatism in their comments (but it's your job to sort that out anyway).
One thing that might make it easier on the participants is to offer to go to them instead of having them come to a lab. It will cost you a lot of time and limit your ability to record everything or have observers watch usability tests but could make the results more relevant and ease recruiting.
Another approach, which I never tried, would be to look for students in the same field (business school students in your case). They won't have much job experience or any idea on the real needs of experienced managers but should at least grasp basic concepts like profit and revenue. Students should be more willing to take some time to participate in a test in exchange for a reasonable incentive.
I am a fan of hallway usability. Grab a person who is walking by and ask them to complete a task. (Give them a treat for playing along). You will find plenty of issues that way.