How do you replace D nuts?special tool?
Yes, to do it correctly where they stay inserted you need the special tool. It basically is a clinchnut tool. It threads into the nut and then the face crimps the shoulder to clinch it into the hole when you squeeze the two arms together on the tool.
where does one buy it?
I purchased my unit off of eBay several years ago, so I am not sure where you should start looking for one.