Find you a model train store.
Forestry is one of the most popular industries to model, so you have them available in a dozen scales from a hundred different manufacturers.
One of the most readily available in the US is Woodland Scenics - and although many people are familiar with their plastic tree kits, they make excellent white metal castings as well of whole trees, tree stumps, and fallen trees:
http://woodlandscenics.woodlandscenics.com/show/Item/S32/page/1
http://woodlandscenics.woodlandscenics.com/show/Item/S31/page/1
http://woodlandscenics.woodlandscenics.com/show/category/LargeTreeKits
For rocks - Shenloanne is on the right track. Just take bits of white metal and smash it up. Lead is safe enough and easiest to get (split shot from big box stores in their sporting goods departments), lead free can be had from many craft stores, but it won't be as malleable or provide as much weight as a counter balance.