You can not catch Squirtle or Wartortle (pre-evolutions of Blastoise) anywhere in Pokemon Gold, Silver or Crystal. They were starter Pokemon from the Red and Blue series, and special bonus Pokemon in the Yellow Pikachu edition game.
You can however, hook Pokemon Yellow, Red and Blue with Silver, Gold and Crystal, with a game link cable; however there are some limitations:
The Pokemon you trade to the earlier games can not be new, they can not know any of the new attacks (those attacks need to be removed by the move deleter), and they can not be holding any items.
When the proper requirements are met trading is possible. So you can get Blastoise, Wartortle or Squirtle from either of the earlier Pokemon games.
If you are currently playing in an emulator in your computer, you can use gameshark codes with most of them. I personally use an emulator called KiGB. Gameshark codes work beautifully.
The code you need to encounter them:
For a Squirtle:
Remember: The area where you use this code affects the level of the Pokemon. If you use it in an area where Pokemon are around level 15-20, your chosen Pokemon will show up around those levels too.