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 7.00 a.m. and after 5.00 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.