I don't know what you define as large breed, but here are some suggestions (very common dog breeds), I don't know if this is your first dog or if you are an experienced dog owner:
Large: Golden Retriever, Lab, German Shepherds, akita, huskies, boxer, bullmastiff, giant schnauzer, standard poodle.
Extra Large: Great Dane, Bernese Mountain Dog, Great Pyreness , Saint Bernards.
If you are a beginner, some great, easy going breeds are golden retrievers, labs, standard poodles, and giant schnauzer. The latter two are especially easy to care for as they do not shed, and require minimal grooming. But with any dog, there are lots of responsibilities, and the key is socialization.
You can check out some local breeders, or even the Ottawa Humane society and other local rescue groups have puppies regularly. Here is a link:
www.petfinder.com
Best of luck finding your dog!