Where are you in your Black or White playthrough?

[Lady Vulpix]

Why catch a Musharna? Catch a Munna and then evolve it. Moon Stones aren't all that rare, I got one very early in the game.

Alternatively, after you beat the Elite 4 for the first time, you can go back to the Dreamyard on Fridays and find a level 50 Musharna.

Warning: Musharna is sloooooooooow. Mine has a Speed-boosting nature, plus a good share of EV-training, and she's still the 3rd slowest among the 12 Pokemon in my main team. She's an HP tank, though. And Special Attack is good if you train it properly.

Curious in-game detail: I recently went to Celestial Tower to blow candles (Special Attack EV-training for a new Ninetales - yes, I'm training 2 Ninetales, I must be crazy or something -) and when I reached the top I found Alder, who talked to me about his past with his first Pokemon partner. Another bit of story which you can easily miss if your journey doesn't take you there.

[Rossymore]

Now that you've done that, you can go face the E4 again and become the Champion.
Though you may get to do that already. I never checked.

[Dark Sage]

Curious in-game detail: I recently went to Celestial Tower to blow candles (Special Attack EV-training for a new Ninetales - yes, I'm training 2 Ninetales, I must be crazy or something -) and when I reached the top I found Alder, who talked to me about his past with his first Pokemon partner. Another bit of story which you can easily miss if your journey doesn't take you there.

Actually, it's hard to miss. You have to go up there and listen to his story if you ever want to challenge him. Until you do that, you won't find him at the Plateau.