Indian nationals can apply online for an ILP for Arunachal Pradesh. Just log on to http://www.arunachalilp.com/index.jsp.
ILP on arrival can also be availed at Naharlagun Railway Station, Gumto Railway Station and Guwahati Asom Paryatan Bhawan and Guwahati LGBI Airport. It usually takes about 30 minutes to get the permit unless there’s a rush. Besides, you can also get ILPs from the offices of the Deputy Commissioners of all 16 districts of Arunachal Pradesh.
Foreign nationals need a Protected Area Permit (PAP) to enter Arunachal Pradesh. The easiest way to get a PAP is through the office of the Deputy Resident Commissioner of Arunachal Pradesh. You can pick up your permit in Guwahati en route to Arunachal. The office is located at a 25- 30 minute bus ride away from Guwahati railway station and is open from 10 am to 5pm (IST). Permits are issued until 2pm (IST)
The office is listed on Google Maps so you can easily find your way there.