I have been making notarial certificates for years. The main reason for doing so is that I am assured that I am complying with the state notary laws.
Another reason is that it saves time if a lot of the wording is pre-printed, such as the venue, my name, and a few other items of information.
Many notaries choose to purchase notarial certificates. But it isn't really necessary. The main thing is that it contains the correct notarial wording, as prescribed by your state.
Also, you can make a notarial certificate in your own handwriting. According to Peter J. Van Alstyne, JD of the Notary Law Institute, and author of Notary Public: Procedures, Laws & Common Sense:
You may need to do this if there isn't enough room on the document for a notary seal and you have run out of pre-printed certificates.
Or you may wish to use your word processor and prepare some notarial certificates in advance. You can print out as many as you want. Here is an example of a notarial certificate that I prepared. Someone called me this morning and wants an Affidavit of True Copy:
As with all notarizations, check your state's notary laws.