stamp paper is NOT required by ACS. In some countries they don't have stamp paper or only use it for certain kinds of documents . A statutory declaration should be a "witnessed statement", i.e. signed by your colleague in front of a notary or somebody similar. The important things are your colleagues signature under a "I make this solemn declaration conscientiously believing the same to be true." statement (or similar) and the signature of the witness, ideally with a stamp of office.