The permits can be bought from:

CHECK POINT  ZTL 
Via Fausta, 397 - Parco Urbano
Open every day Opening Time

ON LINE
Online service NOW  available : http://online.ctservizi.eu/
You can buy tickets ON-LINE by credit cards at least 36 hours before the date of arrival.
BEFORE PURCHASING PLEASE REGISTER YOUR CARD WITH 3D SECURITY  SYSTEM ON YOUR CREDIT CARD WEB-SITE.

PLEASE NOTE
It is compulsory to have a ZTL permit, even for transit before 7 a.m. and after 5 p.m..

In these cases the permit can be purchased in advance at the Check point in Cavallino or at the on-line ticket office.