Patrick,
Try the NHIS website. Althought the race is almost certainly sold out by this time, sometimes folks will contact them with tickets for sale.
Also, many of the online ticket sales websites usually have tickets available, but be prepared to pay the price. I've seen $110 tickets selling for $350 and up on some of those sites.
One other possiblity is the classified ads in the local papers; best bet would probably be the Concord Monitor and the NH Union Leader. Both of those papers have websites as well.
Good Luck.
boB in nh