How do you get pinsur in Pokemon LeafGreen?

You can't get a Pinsur in Pokemon LeafGreen. Pinsur is a Pokemon that was introduced in Generation IV (Diamond, Pearl, Platinum), which came out long after LeafGreen.

If you're looking for a similar Pokemon in LeafGreen, you might consider using Scyther, which learns the move "Fury Cutter", which is a very strong move similar to Pinsir's "X-Scissor".