What can you add to buttercream to make it shiny / shimmer?

Most people add the shimmer after the cake is iced. If you don't have an airbrush you can spray it with Pearl Spray once it's iced.
http://www.pmeartsandcrafts.com/en/lustre-spray/3870-pearl-edible-lustre-spray-100ml.html

I think Wilton just came out with one as well. You can also use shimmer or pearl dust but the application is difficult on buttercream because it's easy to muss up the frosting.

If you have an air brush machine you can spray it with a coat of pearl shimmer. I don't use buttercream, only non dairy whipped topping (Pastry Pride) and it looks lovely.
Also, I think I've seen areosol cans of shimmer on line at many cake supply websites if you don't have an air brush machine.
If you want it shiny, I've also seen online cans of spray edible laquer.
I hope that helps!

Thanks...how many cakes can you spray with that can off earl spray?

Im not sure, I've never used it. I've seen it here:
http://www.thebakerskitchen.net/Shimmer-Edible-Aerosol-Spray.aspx

I use my airbrush machine when I m=need to add shimmer to cakes.