If you can’t find a witness to sign a document… 

Certain documents such as Wills generally require the signatures of witnesses.  It is better if the witness knows the signer, however, sometimes, you just have to get a living breathing human being who is 18 years of age or older.
 
A notary can be a witness. Some states allow notaries to witness as an official Notary act.  But, I have not heard of any state prohibiting a notary from acting as a witness in their capacity as an individual.
 
You can always try to grab a neighbor or a stranger who will be willing to act as a witness.
 
It is good to document the witness’s identification, printed name, phone number and address just in case.  You might need to find that person after the fact, or prove that they really witnessed a real signature.
 
Sometimes, a notary might be instructed to notarize the signature of a witness on a Will.  Although notaries are generally discouraged from notarizing the signatures of the main signer on a will, I have never heard of a notary being discouraged from notarizing the signature of a witness on a Will.

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Can a Notary be a witness?
http://blog.123notary.com/?p=21359

Can a notary witness a will or notarize one?
http://blog.123notary.com/?p=1525

Where do credible witnesses sign the journal?
http://blog.123notary.com/?p=2508