Why do doctors not have nitrous oxide for those undergoing needle sticks?

Regulations: The use of n2o is regulated and controlled by several agencies to assure patient and provider safety and minimize exposure. Have a look at the complexities here: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0049020/.

Not practical: For just the needle pinch ther is topical an aesthetic , vibrating needles and topical patch to numb the area before the needle.

You could: If that is what is keeping you from getting needed work done, I am sure there are dentists willing to use nitrous for as little as as much of the procedure as needed. As long as you are willing to pay the extra cost.