In class we used Kumohada paper, a high quality Japanese paper that is fairly expensive and must be ordered directly from Japan. This is a traditional paper to use with Nihonga and its even texture and consistency accepts the paint well.
However a variety of papers and substrates can be and have been used with Nihonga. Japanese paper is especially good because the Japanese technique for papermaking manipulates the layers of long fibers of the paper pulp in such a way that they interlock, creating a very strong paper. Also the hand-made paper is typically not made with chemicals and is therefore archival. However many western and machine-made papers are also suitable once they have been sized with dhosa.
Here is a video showing how Japanese paper is typically made.
Whatever paper you use, take it out of its packaging once you get to the studio. Paper needs to adjust to the air and environment and should not be tightly rolled or encased in plastic.