I had heard a few years ago that Windows 98 was released to the public domain and could now be downloaded and installed for free, but apparently that is not true.
Not only is it not free, but M$ will not even permit licenses to be sold for those for system builders. The only way you're going to get a copy of an OS that old is used or closeout. The good news? 98SE sells for as low as $35: http://software.pricegrabber.com/windows-family-os/m/54366/
Tom