You have to call the pharmacy at your local Costco and ask if they carry pet meds. If they carry pet medications then likely they carry ProZinc. ProZinc is the name for it.
The last I knew, the pet meds thing was a test initiative centered out of the Chicago office/area of Costco.
If your Costco pharmacy does not carry pets meds please write them and request that they do. Costco is very good about reading customer input.