How can I find an old paper when the usual methods fail? I am trying to find the paper "The resistance of pure mercury at helium temperatures" by Heike Kamerlingh Onnes. It was published in the Communications from the Laboratory of Physics at the University of Leiden in 1911.

This is the first publication of the discovery of superconductivity and it is cited in many texts dealing with the subject. However, I was unsuccessful in finding the actual content of the paper with the methods: My university's library catalogue, Google Scholar and Google. I even tried the catalogue of Leiden University, but no luck there either.

Due to the significance of the paper and the fact that Leiden University still exists, I am fairly certain that the paper is not lost completely and I am just unable to find it.

Are there any other resources I can use to get ahold of this paper?
In the future, your university librarians are MADE for this!  Don't feel ashamed to ask -- there are librarians more into archiving and cataloging, and some more into education and service. 

Give as much information as you currently have, and they'll find it for you AND/OR show you how to find it yourself in the future (if it's accessible via a resource like some of those mentioned above.)