I bought my "official" NH drawbar lock from my official NH dealer. I don't have the receipt handy, but I know it was less than $20. And no shipping cost. If it cost any more, I am sure I would not have bought it, but would have made something on my own.
The part number is shown as SPCDB413ODS. I can't find it in a search at Messick's, but it looks like their Manufacturer search is off right now.
See the attached pic.
Notice my customization - it came with a bolt and nut to hold it on. I thought that was too time consuming to put on and off, so I drilled an eighth-inch hole in the end of the bolt and put in a clip with a washer underneath.