String trees are easy to find, most guitar stores will have several styles in stock. Just drill a small hole a few mm's deep and screw the tree down so it sits evenly between the two strings.
As for the nut, you need to d oa lot of searching a reading here and google is a good bet. You have to make sure the strings are precisely spaced and spaced evenly. And then there is shaping the nut, the slots, and setting the action to the right height. Not hard to do, but takes a little practice and some tools that are more than common hardware store tools. You would probably do best finding the needle files and X-Acto saw at a hobby store. But the files need to be the same size as the string gauges, maybe .001 thicker, but not more than that.