Although 98 licences are no longer available, the wording of the EULA for MS Win 2000 Professional allows the licence to be used to 'downgrade' to other operating systems for which new licences are NO LONGER AVAILABLE!! Funnily enough, the only current OS with this licence 'downgrade' facility is O2K Pro. XP Pro's licence will not allow a downgrade, and neither will Windows Server 2003's licence.
All you have to do is telephone Microsoft and request a 'downgrade' licence for 98 from W2K Pro. You then give them proof of ownership of the W2K Pro, and they will issue you with a product key for 98. This will ONLY allow installation of Windows98 on a machine with W2K Pro installed.