try ebay make sure you get a cdkey with it or it will be no good
also try pricegrabber.com
one of their store fronts
try googling buy windows 98se ( get se ver )
DO NOT FORGET TO GET A CD KEY
IF THIS IS FOR A NEW PC , GO TO THE WEB SITE TO MAKE SURE THEY HAVE DRIVERS FOR THE MAINBOARD, IF IT IS FOR AN OLDER PC..CARRY ON