Osteopontin.Org
Molecular SIBLINGs in Health and Disease

There are at least 27 different commercial suppliers of antibodies that recognize human OPN. A fair number of the antibodies are either the same clone or raised against similar epitopes. An excellent portal into details on these antibodies (and immunoreagents in general) is Linscott's Directory. 

Linscott's Directory of Immunological & Biological Reagents
 
Maine Biotechnology Services, Inc has a number of OPN specific antibodies. They provide a good white paper on OPN as well as an epitope map