Civil Registration records for Buenos Aires are not online.
The info file on Jewish Gen has information on the requesting of records
I had tried once to order a record from the US, but was unsuccessful. I finally paid a researcher to get me the record I needed. I wish you luck.
Secaucus NJ USA
HASSMAN, SONENTHAL, DAUERMAN, LUCHS - Drohobycz, Ukraine
HIRSCHHORN, GOLDSTEIN, BUCHWALD - Dolyna, Ukraine
HIRSCHHORN, STEIN, CAPLAN - Entre Rios & Buenos Aires, Arggentina
ELLMAN, COIRA, MAIDMAN - Minkovtsy, Ukraine
KAGLE, FASS - Ulanow, Poland