It is found on Route 2, but only in the Rustling Grass. It is not that rare, having a 15% encounter rate (although you will find Ekans 50% of the time, so comparatively it is a bit rare). The main note is that it is not found in a normal encounter - you need to check the rustling grass for it.
Also, it may be on other routes aside from Route 2, I don't know. That is just where I found it, and I went on Serebii to confirm its encounter rate.