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Try searching for the title on http://scholar.google.com ― I've found quite a few papers that were behind pay walls using it.

[EDIT] I've not found the papers listed by the OP but other papers in the past.

[EDIT 2] Oops the OP in fact has linked to Google Scholar. My bad. :)

[EDIT 3] Ah in the search result, on the right there should be a link like "[PDF] from ethz.ch".

You can also try using Google's filetype operator, for example:

filetype:pdf "Maximizing the spread of influence through a social network"

And if it's not available online, many researchers will send you a copy of the pdf if you send them a nice email.