This is pure speculation on my part. It seems to me NH would be nuts to have specially designed headlamps made when there are so many standard sizes already available. If I were you I'd remove the burnt out unit and take it to an auto parts store to see if they could cross reference it to a stocked unit. It may be useful to have the NH part number for cross reference.
Beefarmer,
My TC18 owners manual says to use #894 as a replacement headlamp bulb. I am sure your manual will tell you what bulb to use, might even be the same as mine.