From what I remember from my childhood, most snowglobes have a small cap in the base, just like a water-pistol, so you can add water.
Any cracks can be repaired with a small amount of clear silicon sealant, like the stuff around your bath. Buy at B&Q or Home Depot.
If you need to add extra snow I would imagine that flitter would do. Flitter is just like glitter, only with slightly larger particles. You can get hold of that at a craft shop, though in a pinch you could probably use a shredded white plastic bag.
Big Softy
March 2007