Question

I know it's impossible to spot vullaby with trading, but who can I battle to register mandibuzz so I can trade for it later on?

Answer 1

You can battle a Mandibuzz at Victory Road. It is owned by a female Vetaran trainer and it will be at level 55. You need to Surf and use Waterfall from a later area to reach her though.
Source: http://bulbapedia.bulbagarden.net/wiki/Victory_Road_(Black_2_and_White_2)#Entrance_2