You might try dressing up like Jason, and rev up your chain saw, and run towards the tree, yelling, die tree, die! That might scare it!
Sorry, couldn't resist.
Just do like Eddie said, or if you can find some trees with injuries, you should be able to get some that way. I have a lot of trees here that have old scars, and wounds, with hardened sap weeps on them. These are lodge pole pines, firs, and Ponderosa pines.
If you live where there are birch trees, you can put a wad of bark in a can, and set on a small fire, coals, or burner, and the pitch will leach out, and you can pour it into a container, and then dip a stick into that. At least that is what the guy did on you tube. Look up birch pitch. But if you have a hunk of lighter wood, just use that like Eddie said.
Wayne