This should link should provide suitable information: http://www.jagshouse.com/zipMacPlus.html

It describes how to use a Zip drive as a boot disk for a Mac Plus, but the information regarding drivers is there as well as a working download link for the Iomega driver version 4.2. For using the Zip drive in the ordinary manner as removeable media a simple installation of the driver is all that is necessary.

Just download unstuff the driver, plonk it in your Classic's System folder and restart with the Zip drive connected. I've got the driver working with both System 6 & 7 with no problems at all.