How to get reference from lab I worked with, when lab head seems unlikely to help and I was advised not to ask postdoc
One option is to ask the postdoc and professor if the postdoc can write a strong first draft that the professor would be willing to sign (and ideally edit). Most admission committees will probably know that the letter was written by the post-doc, but the fact that it is signed by the professor (in addition to the postdoc) means that the head professor agrees with the assessment. 

This letter will likely be stronger than if only one of the two wrote it because (1) The postdoc will be able to provide detailed examples that the professor doesn't know about (2) The professor's name recognition, through the signature, adds credibility (3) the professor will typically have more experience writing these things and hence he or she may improve the letter by carefully recrafting a few of the post-doc's sentences. Note that (3) and (1) work synergistically. Without the examples provided by the postdoc the professor's experience in letter writing is likely to be less helpful because their letter will sound too generic.

Additionally, this option will also provide the post-doc good feedback for future letter writing (assuming the professor shares her/his edits with the postdoc). So it is really a win-win situation for everyone.