If you are trying to farm nano spores in Warframe, the best places to start are with survival and defense missions on the planets such as Orokin Derelict, Neptune, Saturn, Eris, and Deimos.
Starting with Eris, the best mission you should complete to farm nano spores is Zabala, as the mission is a dark sector, so it has an increased drop rate for resources. So keeping with the theme, the missions Kelashin on Neptune, Akkad on Eris, and Piscians on Saturn all would also be good farming locations.
The reason why these would be good farming locations is that all three are also dark sector missions. So for that reason, we recommend farming them more than others to make the most of the 30 percent increased drop rate for resources.
Related: Does Warframe have Crossplay?
That said, after dark sectors, we recommend trying the mission Titan on Saturn as it is a great place to start for farming nano spores. It's also a decent location for new players, as all you need to do is kill grineers, and the structure of the mission is all-around easy to complete. Killing the many grineer enimies, will also drop the spores as they have a decent drop rate from grineers.