What sort of documents are you looking to have apostilled? If they are US documents, they have to be apostilled by the agency in the US that originally issued the documents. That usually means you have to go back to the state, county or town that produced the document in the first place.

The US embassy here in France has an information sheet on apostille for documents issued in the US: http://www.amb-usa.fr/consul/acs/guide/aposti.pdf
For documents issued in Mexico, try the website of the US Embassy in Mexico. (Use the search function on the embassy website.)

Cheers,
Bev