The RTH Elections site now has an online form you can use to find your electoral riding:
Just enter your postal code into the form and the page will return your riding. It uses the Google Maps API to look up the geocode latitude and longitude coordinates for your postal code, and then uses the Vote.ca API to find the riding for your geocode.
Note: We do not store any data from searches using this tool - neither your postal code nor the query results.
The service can also be accessed via the RTH Elections API.