If you want to add Census shapes to your leaflet map, why not use Census Reporter's data?
To get some idea of what's available, pick a summary level from the menu at the top of the page. Depending on the summary level, the map will zoom to a sort of arbitrarily "close" level. This is because our server will, like a loyal dog, try to fetch all the features at whatever you ask for, but it doesn't deal very well with requests for lots of features. You can experiment a little by zooming out after you pick a small summary level. Of course, while you're doing that, you're sorta grinding our servers, so don't go nuts.
Note: We're working on encapsulating this into something you can do even more simply. Just need to work out a few fine points of default behavior and how much we need to do to actually make it simple.
We would love to refine this service so that it works more flexibly for all su