Hey - I know how you feel!
It is very common for the recommenders to be late and to have to be reminded multiple times. Usually they come through but it takes a few "do you need any more info" type of emails/reminders do the job. Usually it is BETTER to go with your first choice who has had plenty of time to prep. I would attempt to contact the person one more time and if you don't hear back, you can send a "hey, I did not hear from you for a while. I am really serious about bschool and though I appreciate your effort and help with the recommendation, I will ask someone else. I value our relationship and I hope it has not put a strain on it. No, go screw yourself ahole."
The last part is optional
As an FYI:
Traditionally, college/academic references are not as encouraged due to the fact that business school is about a professional/leadership setting, and professors don't always have enough exposure in a large class to observe that. You are also in a college setting where your responsibilities are limited; consequences are muted, anyway, you get the idea.
You want to get a recommendation from someone who knows you well and can talk about you in a positive light. If they do not know you very well, the recommender needs to be briefed/helped with any of the questions they may not know answers to (some questions are purposefully insightful to see if the recommender knows you well enough).
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!
Get the best GMAT Prep Resources with GMAT Club Premium Membership