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Singers must have the piano-vocal or chorus score of the G. Schirmer Edition (in English). Check out our Tips on Music Sources.
Non-singing Conductors need an orchestral score of any edition and size (full or miniature). Dover is a good edition. It is full sized, printed nicely, and inexpensive. Kalmus is also fine. Recommendation: the Kalmus miniature score is extremely small. Better to get the Dover full-sized score for not much more money.
Singing Conductors need both an orchestral score (as described above) for the classes and workshops, and the G. Schirmer piano-vocal score for when they sing in the chorus.
This is readily available and inexpensive (Tower Records link below) http://www.towerrecords.com/product.asp?pfid=1180409.