Instructions
Travel to Celadon City. Celadon City is in the center of your map, directly above the river.
Go to the cafe and speak to the man sitting at the table. He tells you about the city's casino and gives you a Coin Case.
Go to the casino, called Celadon Game Corner, on the north side of the city.
Speak to the woman in the front of the casino. Exchange your Poke Dollars for casino coins and begin playing. You need at least 9,999 coins to acquire a Porygon. Depending on your luck, accumulating so many coins may take an extensive period of time.
Exit the casino and go to the building to the right of it. Speak to the woman at the counter and select "Redeem Coins." Select "Porygon" from the menu. Porygon is added to your inventory and the coins are removed.