TM46 (Thief) is found nearby the Galactic Building in Eterna City in DPPt. Requiring Cut, simply use the HM move on a nearby tree and head down the path that opens up from it.
Go to the Galactic Eterna Building (the black building to the north of the Pokemon Center). Use Cut on the bush in the northeast part of the lot to access a path that will take you to TM46, Thief.
