The easiest way is at the airport: after landing at Bandaranaike International Airport, visit the 24/7 DMT counter in the arrivals area (next to SPAR supermarket). Bring your passport, original full driving license, 2 passport photos (white background), and LKR cash. Fees: LKR 15,000 (1 month), LKR 21,000 (2 months), or LKR 30,000 (up to 6 months). You receive the permit on the spot. Additionally, getting an IDP before you travel from your local auto association (AAA, AA, ADAC, etc.) for $15-30 USD is recommended as it simplifies things and serves as an English translation of your license.