Milky Way position on the sky
There is a very nice project called d3-celestial by Olaf Frohn on github. In contains a data file describing the Milky Way as polygons, see here. A demo showing this Milky Way can be found here. And even better, the source for this data is cited, pointing to the Milky Way Outline Catalog by Jose R. Vieira.

Depending on your project, the json format from d3-celestial might be easier to read than the one from Jose R. Vieira.
Note that you don't have to worry about these "contours" moving on a time scale of hundred years, but this is another question.