I’ve looked on […] eBay where I could buy a slide rule but haven’t been able to find one.

What have on earth have you been typing in for search terms?

* https://www.ebay.ca/sch/i.html?_nkw=k%2Be+4081

* https://www.ebay.ca/sch/i.html?_nkw=k%2Be+4083

* https://www.ebay.com/sch/i.html?_nkw=aristo+slide+rule

* https://www.ebay.ca/sch/i.html?_nkw=post+versalog+ii

* https://www.ebay.ca/sch/i.html?_nkw=sun+hemmi+262