There are a few Nikon F (AF-S) to Sony E/FE mount adapters out there, mostly from chinese companies (for example, the Viltrox NF E1) but I haven't seen really positive results from them:
- some have limitations like AF-S only
- many focus hunt
- some lock up the camera every now and then.
There is also the chance that the image quality might be poorer on the A9 compared to native lenses (due to misalignment in the new mount pairs, different filter thickness on the sensors, different microlenses).
Overall, I would say that, if you have the A9 and want to play around with some Nikon lenses, an adapter is worth the price.
But I wouldn't buy an A9 just to use with Nikon lenses...
P.S: funny enough, Canon lenses seem to perform better on Sony than Nikon ones, and there is even a Sigma adapter for them.