I don't read DCs, but Marvel has sets of DVDs in which you can buy entire runs (frequently from issue 1 to aprox. 2000-2008) in CBZ or CBR format. You're going to be hard pressed to find these for free from a legal source though. They run anywhere from $60 for lower print runs to $180 for things like the full Avengers run (once again, this is all marvel, rather than DC.) I've used a friend's version of the Ironman Anthology and they're VERY nice HD quality pages.
You might be lucky to find a used copy of a Batman Anthology that includes the old Detective Comics and Golden age books for cheap, but I doubt you'll find them from a legal website.
Edit: I should also note that a lot of Golden Age comic books haven't really aged well as far as art and story is concerned. If you're determined to start from the beginning, find a good universe reboot point (Lord knows DCs done enough of them) and start from there imo.
[Edited by insomniac, 1/19/2013 2:05:14 PM]
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Finding good, free comics is not that easy unfortunately. I will see if maybe I can find something tomorrow since its 1:45am here right now, and I am very tired.
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