I got it in Pokemon Pearl. I was breeding Togepis and trying to get a Togekiss. I really don't know where I got it from in the first place though. Must have been during a battle, trying to level up Togepi. But one of the Togepis got it randomly.
WaiT
I've never gotten it... ever... and you've gotten it multiple times?
I must be quite unlucky. Either that, or you're quite lucky!
To answer the poll, I would love a Houndoom with Pokérus