The Motor Carrier Services, Special Permit Sections issues the Oversize/Overweight permits. You can order permits online through a permitting service with an additional processing fee, by mail to:

Permit Office

New Hampshire Department of Transportation
PO Box 483, 7 Hazen Drive
Concord, NH 03302-0483
Permits by telephone: 603.271-3734
Monday – Friday 6:00 a.m. until 4:30 p.m.

See more information: http://www.nh.gov/dot/org/operations/highwaymaintenance/overhaul/index.htm.