I'm not sure what you've already tried, but here is where you should be able to get the driver:
ftp://ftp.sco.com/pub/openserver5/pub/drivers/OSR506/btld/ad160
To install these drivers, download the btld image and the accompanying readme file and follow the step-by-step instructions.
Hopefully, you've got another UNIX system operational, which makes it easier to generate a diskette from the downloaded driver:
dd if=driver_file of=/dev/rfd0135ds18 bs=18k
Once the driver is created on floppy, boot the system on the installation media, and when you are at the "boot" prompt, specify that you want to link a new driver in.
boot: defbootstr link=ad160
This should be covered in the docs on that FTP site.