Okay, so according to Serebii's page, you fight Silver seven times. You fight him at:
1 - Cherrygrove City
2 - Azalea Town
3 - Burned Tower
4 - Goldenrod Underground
5 - Victory Road
6 - Mt. Moon
6.5 - Dragon's Den (Double Battle partner)
7 - Pokémon League
Now, the seventh time you fight him is actually in the Pokémon League building, not on Indigo Plateau specifically. It also only happens at a certain time.
>On Mondays & Wednesdays however, when you enter the Pokémon League, you will get challenged by your rival before you enter. He has been training and wants to test his team out
Try looking for him on a Monday or Wednesday.