Can anyone tell me if these really bling. Can it be used on buttercream or just fondant. Where is the best place to purchase?

I don't know the answer to your questions but I'll give you a bump. Maybe someone will answer soon.

Fondant source.

They don't shine and glisten like real diamonds. Just imagine clear boiled sweets, they have a similar look, I don't know if you could add edible glitter to the sugar before pouring (if you made your own).

The key to using them and keeping them clear is to keep moisture away. If moisture gets to them they go cloudy. You are supposed to add them to your cake at the latest possible time (ie right when you finish setting up). To revive them you can give them a light coat of vegetable oil but this doesn't last that long.

Where are you so we can advise on the best supplier?

thanks for the help and I am located in Louisiana

Fondant source in in Florida. They have the best prices.