Like the Johto gym leaders, you have to acquire their number and then schedule for a rematch. This counts as the same quality for the Kanto gym leaders.
Brock - Found in Diglett Cave everyday in the afternoon, accepts rematches on Saturday nights
Misty - Found on Route 25 every Friday night, accepts rematches on Wednesday mornings
Lt. Surge - Found outside the Power Plant everyday; at 9-11am, accepts rematches on Friday mornings
Erika - Found near the Celadon City Fountain on Sunday in the afternoon, accepts rematches on Sunday mornings
Janine (Koga's dad, which is part of the Elite Four) - Found outside the Victory Road Reception Gate every evening, accepts rematches on Mondays, daytime
Sabrina - Found near the S.S. Aqua every Friday night, accepts rematches on Sunday afternoons
Blaine - Found outside the Cinnabar Island Pokmon Center on Tuesday mornings - accepts rematches on Tuesday afternoons
Green - Show a Pokmon that has maxed happiness to Daisy Oak (his sister), after that Pokmon was massaged at least 5 times, accepts rematches on Saturday Nights
Oh, and this is my first cheat. Hope it's the best.