Where to get help when we can't understand something done in a paper? I often face the following situation:

I am reading a technical paper, and I can't figure out a calculation shown in the paper, whose details are not given. There is no reference in the paper about the exact calculations. It is not possible to get an answer from the author(s), either they are dead or they don't answer even if I did (even if they did, it takes a lot of time for them to respond). I can't write to my advisor; for him to help, he also needs to read the paper and practically this is neither ideal nor realizable in a short amount of time. I can't write to my friend because I don't have any friend who works in the same field as I do. I won't ask the question in one of the stack exchange websites, because it is often the question is too field-specific and technical that I don't receive any helpful answer in a considerably short amount of time (assuming that I will get a helpful answer, which is not the case most of the time).

So, in the end, I try and try to understand what the heck author did in that part of the article that I couldn't understand, and it takes a considerable amount of time for me to figure that out (if I can).

Question:
Assuming that I am not the only one who faces such problems, where can we get help in such situations?

Note: I am working in a field of Physics.
Assuming that you've exhausted a reasonable degree of effort to try and understand what the author is doing, you can always just contact the author directly. There is usually contact information on the paper for the corresponding author. 

There have been a number of papers that I have read and needed to reach out to the authors for to get a better understanding of what is going on in their paper. Professors are people too. As long as you have demonstrated a reasonable degree of effort prior to reaching out, asking for some clarification from them is a reasonable thing to do. 

In the case of an author having passed away... then you will want to communicate this to your supervisor and explain why you need help to understand the paper. If you cannot figure it out for yourself, do not have any peers you can talk to, or are unable to contact the original author, then you have your supervisor as a work colleague.  

If that all fails, then you will just have to try and drudge through understanding the computation. While I understand you want the answer quickly, all things worth while do not come quickly, and you'll need to cultivate and demonstrate resilience in the face of that struggle.