Check on eBay. The IBM eServer 325 is a good series, and can be found for between $50-$70 dollars shipped. I don't know if it supports Windows 98SE, but you could always install Ubuntu (
http://www.ubuntu.com/) and then use VirtualBox (
http://www.virtualbox.org/) and virtualize the windows 98 boxes, just passing the network cards directly to the 98SE OS.
--Nick