New York State doesn't have death certificates online or for general lookup as they are confidential under state law. They must be applied for and purchased. Death certificates are available to anyone if the year of death is 50 or more years ago. More recent ones are restricted to immediate family members who provide proof of relationship. You can contact the Ontario Town Clerk at (315) 524-3441 and ask either for Debra DeMinck (town clerk) or Shelley LaRocca (deputy clerk). One of them should be able to help you. Death certificates for genealogical purposes through New York State are normally $22.00, but the Town of Ontario may have a cheaper price (possibly as low as $11.00) although many other locations happen to go with the state price. I haven't gotten any certificates from them at all, so I'm not exactly sure what they charge.
The New York State Death Index is online, which only shows the date of death and location of death for the decedent. It should be certificate #6541 for Sophia Warren, Town of Ontario.