No you can't sorry. The only way to get it is in the Yellow Forest route of the Pokewalker or whatever the route is called.
I'm not so sure because I havent seen one and I'm up to the elite four
Don't be stupid that's not a pokeball that's a held item and the only way to get it is in yellow forest
No but you can get from yellow forest or from a random pikachu(5% chance) catch a pikachu then put it first in party and go to viridian forest and keep catching
No you can't. Sorry! I know this because I have already beaten the game and have been to all the Pokemarts. But there is a huge selection of other Pokeballs to choose from!