Superman Dailies 1942-1946

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The only free one I know of is the Toronto Star, that offers access to 1945 for free. 
http://thestar.pagesofthepast.ca/Default.asp
The rest was through subscription services offered through universities and the like. 

The other trick is that papers didn't carry the entire strip, and often the ones that did missed days, or there was poor archiving of a certain day or week.  Still trying to finish the last 2 years of the newspaper....