First, La Rochelle is not located in Manche, Basse-Normandie. (That is like saying "Dallas in Massachusetts" or "Vancouver in Québec.") Either your city or your departement in France is not correct. You would benefit by looking at your source again to get the correct information.
If your ancestor is from La Rochelle, the record may be easily available online at the departemental archives of Charente-Maritime. There are seven large Catholic churches in La Rochelle to search, each with their own collections of parish registers. Access to the digitized records is free. The website is available here: http://charente-maritime.fr/CG17/jcms/cg17_47603/les-archive...
If your ancestors are from Manche, Basse-Normandie, the Catholic parish registers that survive have been digitized and are available online at FamilySearch.org. (You will need to know the town or parish where your ancestor lived.) Access to the digitized records is free. The website is available here:https://www.familysearch.org/
There are many places you can access the Drouin and Tanguy records online. If you have a membership to Ancestry.com you already have access to them. Simply go to the Ancestry Card Catalogue, then, type in « Drouin » or « Tanguay » in Search Titles, then click Search, and choose your collection.
If you are near a good library, the Drouin and Tanguay books are probably there for you to search as well, and if not, you can get them through inter-library loans in the states or Canada. If you live in the UK or Australia, they may be found in libraries of largest cities. A librarian can help you.
The Canadian records you are looking for are also indexed and available online at Le PRDH (Le Programme de recherche en démographie historique), Université de Montréal. You can access their terrific database here (French or English):http://www.genealogie.umontreal.ca/fr/
Regarding Paris : More than 70% of the parish registers for the city of Paris (only) before 1860 were destroyed in fires by the Communards in 1871. Unfortunately, all of the parish records for Saint-Benoît in Paris were lost and none survive. Your Paris information must come from a Canadian marriage or death record you can find in Drouin at Ancestry.com, or the PRDH.
You now have access to all of the files you were looking for, to, as you say, « tap into » !
Cheers to you !
Bonne quête !