Certain branches of the 60 bus go to San Isidro via Plaza Italia in Palermo; the route you need is 'Alto Tigre por Bajo San Isidro'; check with the driver.
The Tigre branch of the Mitre line train from Retiro stops at San Isidro and Beccar stations (with/without SUBE card AR$5/12, 35 minutes).
The most scenic way to reach San Isidro is via the Tren de la Costa, whose southernmost station (Maipú) is in the suburb of Olivos. The Mitre train from Retiro will take you to Maipú (get off at Mitre station at the end of the line and cross the pedestrian bridge to Maipú).
You can also reach Maipú station on the 152 bus, which stops at Plaza Italia in Palermo.