Which canadian law school offers a LLM in tax law ?

Most of them do - that is, most will have a faculty member who specializes in Tax who might potentially be your supervisor. However, I would specifically recommend you consider the University of Western Ontario (UWO) - Daniel Sandler and Tim Edgar are two of the top people in Tax in Canada. In the LLB program there is a specialzation in tax and I think (but am not sure) Daniel Sandler is the Associate Dean of the Grad Program. UWO only started offerring LLMs a few years ago but that should not deter you from going there as I said above, UWO has two of the top tax profs in Canada.