If you've priced bottle brush trees with mercury balls added, they can be very costly. You can do your own of course--here are two links where you can get mercury glass beads that won't break the bank
Vintage mercury glass beads http://www.papiervalise.com/Provisions/ ... Beads.html
Tiny vintage mercury glass beads http://www.papiervalise.com/Provisions/provisions.html

I've used tiny balls from a string of ornament-garland I got at a thrift shop. They were multi-colored, and multi-sized, though most were very small.