mention the sources that provide information …

There are many sources of information regarding Harappan Civilization. Out of them, the most important were seals, script, excavations etc.
SEALS:

• The Harappan seals are the most important sources for reconstructing the History of Harappa.
• The seals were unique feature of Harapppan civilization.
• The seals were made up of steatite and they were in square and round shaped.
• Seals were mainly used for the purpose of trade.
• The seals were pictures with animals, plants, etc.
• One of the most important seals was the seal of Pashupati.

SCRIPT:

• The script of Harappan Civilization is another important source.
• Though the script was not deciphered, it is one of the sources for Harappan Civilization.
• The script of Harappan Civilization was not alphabetical instead it was pictographical.
• There were nearly 126 characters in the script of Harappa.
• The style of Harapppan script was bostrofedon style.

The other sources for reconstructing the history of Harapppan civilization are Terracota figurines, Great Bath, Granaries etc.