Different programs will have different expectations. It seems like a good idea to check with the schools you are targeting. Maybe start off by asking people for LinkedIn recommendations: that's an easy way to test the water without raising the question of an MBA. You can ask clients and formers managers for that. Manager known your strengths and weaknesses more deeply and are more likely to understand what an MBA will do for you. That is why schools look for managerial endorsement.

Why aren't you sure about asking clients? This seems like a great idea, especially if you have worked closely with them and can speak to your potentials. But yes, ideally, you would also have something from your current manager. And I don't believe that letters from former professors are absolutely mandatory, and in fact, some schools value them less than those from people who you have had a working relationship with. This is from Stanford's admissions page, for instance:

[quote]We love professors — we are a school, after all — but rarely are faculty members the best choices for MBA recommendations. If you worked with a faculty member outside the classroom, perhaps as a teaching assistant or on an independent research opportunity, then that professor might be in a position to write a helpful recommendation. Still, you need to think carefully about whether that person can address the questions we ask in the recommendation form.[/quote]

Thank you both for your advice!

On the topic of letters from clients: do you think that schools will look at these less fondly than letters from managers? I haven't seen a lot of discussions about people using these kinds of letters in the applications.

Yes, the person who has supervised you the most over the last year or two is always ideal. Anyone else might not know your weaknesses, and is obviously cherry-picked.

Got it, thanks again.