Why do you need to purchase it from an authorised agency?
Carrying foreign currency to a foreign country is not a normal phenomenon and needs proper channel of conversion. Currency without proper bills will be confiscated by the custom officials at the airport. DO NOT HAVE SUCH EMBARRASSING SITUATIONS.
madura travel service (P) Ltd
25-3, Gandhi Irwin Road, Egmore, Chennai, Tamilnadu 600008, India