Go to Haemar's Shame, set difficulty to Novice, and let the vampires attack you until you catch it.

You might want to take health potions just in case.
I'm also fond of North Shriekwind Bastion just NW of Falkreath. There's only two Vampires in there; The first being a low-level one just inside the entrance to the dungeon, the second Vampire is a Master Vampire alllllllllll the way at the end of the dungeon.
Since you can only catch the disease when a Vampire uses their "life-drain" effect (and it's only a 10% chance
per use by the Vampire), I usuall save just after entering the dungeon. Then I run forward and let the Vamp hit me with it's red death ray, and if I don't get the message saying I contracted it, I'll put a wall between me and the Vampire to break the stream of the Vampire's power. Then I reemerge and let it use it on me again, repeat until you get sick. lol
If this method seems like it isn't working, then just reload from the save I had you make earlier and try again until you get it.
