Anywhere huge crowds are gathering, or there are people are streaming in and out of a building, or people are waiting.
However, you need to have access to the foot-traffic. So if it's private property, you'll either need permission of the property-owner or event-organizer, or you'll need to have a public egress, i.e. a public sidewalk, next to the door. Not all locations have this, but all supermarkets and all public buildings (library, town hall, etc) do. Please take a look, near the bottom of the How to Petition page, for more information and some important documents regarding petitioners' rights.
Let is know if you have know a good source of event-listing for us to check out. Here's some sources we know about. We plan to list some big events on our Calendar page, but we can use your help.
Festivals can be a huge source of signatures. While you can gather signatures anywhere, we recommend going to concerts with artists who are progressive thinkers. We'll be posting the huge concerts at Mansfield that we'll want large teams canvassing.
The Lowell Sun Event Listings