The word Canada was used to designate political bodies starting in 1791 with the splitting of the province of Québec in Upper Canada (ON) and Lower Canada (QC). So probably a bunch of official mentions there in governmental documents. Then in the 1820s - 1830s you have the Parti Canadien (later Parti Patriote) in Lower Canada which begins to amass political momentum leading to the rebellions of Upper and Lower Canada, which, broadly speaking, aimed to create independent Republics (sometimes referred to as the Republic of Canada or simply Canada). So probably many manifestoes, newspaper articles, reports and books talk about these things during the early 1800s. This is just my take on it based on my reflection from my couch. There may be factors I did not consider.