You can use https://www.inaturalist.org/pages/api+reference#get-places
https://www.inaturalist.org/places.json?ancestor_id=6883
but note that community curated places in Ontario may not have Ontario set as an ancestor. You might be able to get more from processing http://www.inaturalist.org/places/inaturalist-places.csv.zip, or you can use the list of community places on a relevant observation (this is essentially just the results of a call to https://api.inaturalist.org/v1/places/nearby):
