From Munich main station the best option would be to go to a Lidl supermarket and to buy a Lidl mobile SIM, which actually is a second brand of Fonic and most importantly is a bit cheaper than Fonic.
For more information on Lidl mobile read
LIDL mobile - Prepaid Wireless Internet Access
There are two Lidl supermarkets close to Munich main station:
Both are opened daily except for Sunday until 20h.