In French Catholic Quebec vital records were only kept by the churches right up until modern times. The records available are baptism, marriage, and burial. A large number of baptismal records are available on microfilm through Family History Centers as are some burial records. Marriages are widely available through the Drouin microfilms, CDs from various publishers, and pre-1800 from PRDH online by subscription (http://www.genealogie.umontreal.ca/
). You might want to check the Canada GenWeb site to see what they have for this county and also Google to see if L'Islet has a website. Your best chance of finding what you need is microfilm and your second best is to post your queries here.