Because the CT5803 was often an OEM part, original drivers were frequently hosted by system manufacturers like Gateway or Dell. Today, you can find them through community archives: Sound Blaster 16 issue... - VOGONS
If the PCI 128 drivers fail, look for the generic Creative Audio PCI drivers for the ES1373 chipset.
Solved: Finding the CT5803 Driver for Windows 98 (Retro Gaming Build)
: While it supports Windows 95 through XP, many users find it most stable and functional under Windows 98 . Where to Download the Driver