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While a quick search shows a few around the Colonia Roma area, if you want to be sure about the legality of one that's recognised in the United States as well (and I don't know what the rules are there), then the best bet may be to use the Notary Services available at the US Embassy in Mexico City.
Notarial Services and Appointments
Notary services are by appointment only from 2:00 p.m. to 3:00 p.m. To make an appointment, click here. If you have multiple documents to be notarized you only require one appointment for that day. There is no fee to make an appointment, but there is a $50 USD fee for each document to be notarized that is paid at the Embassy. Please arrive 20 minutes prior to your appointment to allow sufficient time to pass through Embassy security.
I've quoted the introduction, but to make an appointment you can do so on that page, and it'll also tell you what to bring.
The address of the embassy is:
American Embassy in Mexico City Paseo de la Reforma 305 Colonia Cuauhtemoc 06500 Mexico, D.F.
More contact information is available on their site.