On top of the back-left grave in Goodsprings Cemetery (near the water tower), at the base of the grave marker. The grave is the only one in the area with rubble on top; the snow globe can be found amongst it.
In the cocktail lounge of the Lucky 38 Casino. The snow globe is on the counter "behind" the middle cash register with a green lamp next to it (you must first speak with Mr. House to gain access to this area).
In the Vault 21 Hotel, on the nightstand in an average-locked room (Sarah's) in the northeast corner of Vault 21, next to the buyable room. The room also includes some wardrobes with caps in them. You can pickpocket Sarah for the keys if necessary.
Located near Dean's Secret Stash in the upper levels of Salida del Sol North. From the southern-most entrance to Salida del Sol South (inside a house in Salida del Sol North), proceed down the stairway and out to the street, then look behind you for the entrance hole in the wall above. This area can be reached by going up the stairs located around the corner to the southeast and following the rooftops back towards the hole.