I can't access a reference of an article I am reviewing: what shall I do?
While online access is very useful, it is not the only access. Every university I have been affiliated has provided an ILL service either at no cost or a nominal charge (e.g., a $1 processing charge). US University libraries can make requests to both the Library of Congress and the National Library of Medicine. I am less familiar with the ILL system in the UK/Britain, but I believe ILL requests can be made, possibly through a university, to the British Library.

There are of course some articles/books that are truly hard to find (e.g., not held by the LoC with deceased authors). In these cases, it seems reasonable to request a copy through the handling editor. You should never contact the authors of a manuscript under review directly.