Where do you get scyther on Pokemon Red version?

You can't actually get Scyther in Pokemon Red Version.

Scyther is only found in Pokemon Blue and Yellow Versions.

* In Pokemon Blue and Yellow: Scyther can be found in Route 24, which you'll encounter after getting the second badge. It's a relatively common encounter there.

If you really want a Scyther in your Pokemon Red game, you'll need to trade with someone who has one from Blue or Yellow.