Take a look here.
http://www.ebay.com/sch/Desktops-AllInOnes-/171957/i.html?Operating%2520System=Windows%252098&LH_ItemCondition=3000&_dmpt=Desktop_PCs&LH_PrefLoc=1
Picked this out of the lot....
Type: Posts; User: RickyTick
Take a look here.
http://www.ebay.com/sch/Desktops-AllInOnes-/171957/i.html?Operating%2520System=Windows%252098&LH_ItemCondition=3000&_dmpt=Desktop_PCs&LH_PrefLoc=1
Picked this out of the lot....
My first thought was to consider just buying an old system on ebay. I just can't imagine building a new system just to run Win98.