Mikhail,
Considering this is post-bacc, I'm assuming your issue is taking enough classes as a post-bacc to gain access to a committee letter.
As a post-bacc, one tends to have a greater story to tell. A committee letter can significantly help in telling that story. I seriously thought about doing Harvard's post-bacc program because of the caliber and the existence of a committee letter. After talking with Dr. Fixsen (program director and summer bio professor), he said to take the classes where you will be most happy. I ended up taking his summer bio class. Your story can be told in other areas, aka Personal Statement.
Northwestern, where I went for undergrad and 3 out of 4 post-bacc pre-med classes, has a recommendation file service. I feel the most important thing is to find professors and physicians who know you well. I believe if you work really hard with a professor and he knows you well, yet you received a B+ in the class. He or she is still a great one to ask. The work ethic and personal qualities will be written in the letter. As a post-bacc, your science professors will know recently. Finding a non-science professor could prove difficult if you graduated a long-time ago or never really had a strong relationship with one. Fortunately, I had 5 classes and wrote a honors thesis with the same professor. Lastly, a good compilation of recs will have a medical doctor recommendation(either clinical or research).
Undergrads are faced with a different challenge because most of their science classes are taken 2, maybe 3, years before beginning the application process. Maintaining relationships with a stop-in visit once in a while can help foster a better letter. Also, asking T.A.s to write some words to the professor can help. A lot of times they know you better than the professor.
Additionally, as a post-bac, the science classes tend to have 60 students instead of 300, allowing for a student to better know the professor. I would believe a professor who's forgotten all about you after a few months probably wouldn't have been a good one to ask in the first place.
Lastly, remember recommendations are important but not the end of the world. The goal is to get strong ones. But strong recommendations aren't going to overcome a 3.0 and a 26. Talk to older students and what they've heard about different professors. Remember, medical schools receive recs from the same professor for a number of applicants. You want yours to stand out from mutual letter applicants.
Not that I don't believe what is being posted here.