You can get the old versions from http://www.brywright.co.uk/downloads/files/index.php?dir=natural-selection/install/
The oldest ones would predate Steam I think so if they won't install you'll need to extract them with something like Universal Extractor (but I think you should be able to give it a directory to install to). You can then copy the ns/ directory they install to into steamapps/common/half-life/
Not sure where to put the .dem files - http://twhl.info/forums.php is pretty alive and across that kind of thing so you could try asking there.
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The oldest ones would predate Steam I think so if they won't install you'll need to extract them with something like Universal Extractor (but I think you should be able to give it a directory to install to). You can then copy the ns/ directory they install to into steamapps/common/half-life/
Not sure where to put the .dem files - http://twhl.info/forums.php is pretty alive and across that kind of thing so you could try asking there.