How do you get cyndaquil in Pokemon LeafGreen?

You can't! Cyndaquil are not found in the region and Pokemon FireRed/LeafGreen are unable to trade with Gold Silver Crystal HeartGold or SoulSilver.

NEW ADDED-(takreader)

Actually, it is possible. If you have a link up cable for Gameboy or Gameboy Advanced SP AND a Old Old Old Gold or Silver version you are in very good luck. Because if you need a totodile or any of the starter Pokemon in that game for FireRed/LeafGreen you can use the link up cable to trade a Pokemon over to your Gold/Silver and get the starter Pokemon you need! you catch rattata and give it a rare candy and it transforms into cyndaquil