You should be able to find some at any craft store that sells baking supplies if you're located in the US. But you do specifically want Luster Dust that comes in a big tall tube and not the stubby sort of 'pot' that's labeled as Wilton brand. Wilton makes a lot of good stuff, but their shimmer dust stuff clumps in liquid and doesn't produce the same effect.
You should be able to find some at any craft store that sells baking supplies if you're located in the US. But you do specifically want Luster Dust that comes in a big tall tube and not the stubby sort of 'pot' that's labeled as Wilton brand. Wilton makes a lot of good stuff, but their shimmer dust stuff clumps in liquid and doesn't produce the same effect.