(I'd get an earlier sprite of Pikachu but they all have grey surrounding it. :P)
Pikachu is available in Viridian Forest, the earliest place to get a Flash Pokemon.
Other Pokemon include Pikachu, Raichu, Clefairy, Clefable, Jigglypuff, Wigglytuff, Abra, Kadabra, Alakazam, Slowpoke, Slowbro, Drowzee, Hypno, Voltorb, Electrode, Chansey, Staryu, Starmie, Mr. Mime, Electabuzz, Jolteon, Porygon, Zapdos, Mewtwo, and Mew.
Hope I helped. :)