My Phantom 2 Vision + flipped over into the snow upon landing.  It got snow on the motor.  Now one motor does not work properly.  Should I replace the motor and card?  How do I get replacement parts?

What do you mean by "does not work properly"? Did you give the motor 48 hours time to dry out completely? You might have a damaged motor or/and a bad ESC, hard to tell.  Both could be Purchased at DroneParts for example or ebay...

If you get the spare parts, you have to make sure that you're getting the same version of motor and ESC (Phantom 2/Vision ESCs are not differentiated by the color of the LED. You can use this ESC for any of the four motors) than the other 3 existing ones.