Here are a couple of different options for you to try. One option would be to take your brother's death certificate, the car, and the car's ownership papers to the dealership where you'd like to sell the vehicle, and ask them whether they can help you obtain the vehicle selling package that they would need. If it is a relatively large dealership, it is quite possibly not the first time they have had to deal with a similar situation. If they can manage things for you, then that'll be the easiest. On the other hand, if they say they can't do it without a certificate of appointment from the Court, then you still have other options.
There are several ways to get free legal services in Ontario. If all you need is a letter on Legal Letterhead, then I would suggest that you try a local legal clinic, some examples are:
The third possibility is through the Law Society's legal referral service. The lawyer they refer you to will provide up to half an hour service for free. If you start by telling them that Service Ontario told you that you needed a letter on Letterhead, and you bring all the information needed to do that letter including the Death Certificate for your brother, your ID, and the information on the car, it is quite likely that a lawyer would do the letter as part of that half hour service.
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My answer here contains only general legal information and not legal advice. No solicitor/client relationship has been created by this communication.