the lesson of this story is to build good relationships with the people around you, not only a working one, maybe try to know people on a more personal level. believe me, it really helps. no man is an island as they say.
what you should do is to try to find intructors who are "nice" and "friendly" , the ones who are good-natured and get along with everyone. they probably get asked a lot by other students. if you're still shy to do this, why not ask a friend to go with you that way the two of you can both get reference letters.
in the future, try to open up more to others (your future collegues, supervisors, etc.), because aside from giving you reference letters, employers usually do a character check, and it would be nice if the people around you could put in a good word for you.
don't be shy. you NEED that letter. it's a tough world out there you need to beef up your resume.