What to do in case when advisor refuses to provide a reference letter for a postdoc position? What to do in case when PhD advisor rejects everytime (by his own choice) a request for reference letter for a possible post doc position? Majority of decent post doc positons requires PhD advisor's reference letter.

In the industry, a company cannot dare to hold the experience letter for an employee. But in academia, the situation is not the same. In such a scenario what is a possible solution? Is it that till the time he is ready to provide a reference letter I have to wait? or is there some way out of this?
I do not know your exact field but in most fields you could put someone else from the same lab (e.g. a senior postdoc you worked with) stating that this person was the day-to-day supervisor and the official supervisor is too busy.

But apart from this way out you should also clarify why your supervisor does not want to write the letter. No time? Different scientific opinion on some of your results? He thinks you are not good enough for academia (sounds mean but some supervisors have that opinion on some of their students)? You could become a competition as you know key results from the lab? For each of these points solutions can be found but I would need more details ...