The home page of the Polish Virtual Library has a good search interface and the older issues of the classical Polish journals such as Fund. Math., Studia Math., etc. as well as the monograph series are available for free more or less in their entirety. Here's a link to the old repository which is easy to browse, but no longer updated.
Here's a direct link to Hopf's paper you're looking for and here's the link to the library's details on the article.
Here's the freely available Zentralblatt entry containing a summary of the paper (in German).
A few generic remarks on locating papers online:
As a rule, when looking for a specific published paper, I just search for the name of the journal which quickly leads me to the journal's home page. The article (if available) usually is only a few clicks away (modulo subscription). Looking for author and title is less likely to lead to the paper (especially if it is old and famous) because you're more often served papers that cite the paper than the paper itself. Of course, if the paper is relatively new (newer than mid-90ies, say), chances are that the paper can be found somewhere on the author's homepage or on the arXiv, see also the UC Davis front end of arXiv.
For papers that were published after around 1940 MathSciNet often leads to the paper using the “article” or “journal” link it provides. Unfortunately, MathSciNet it isn't fully linked with the journals apart from those of the major publishers and it needs a subscription for searching and viewing the entries, but most universities (or at least the libraries) do have access.
Alternatively, you can use the Zentralblatt which also works (in a rather limited way) without subscription, but if you have full bibliographical information you can find the papers and their reviews there.