The ZPV (zentrale Personenverwaltung) number is the unique number that identifies you as tax payer and is therefore managed by the finance department of your canton.

It's usually on the tax form that you receive at home under the field "referenz".
If you're taxed at the source, then you have to ask for this number from your tax/finance department of your canton (that's how i got it....it was 10 years ago)