When you download the free trial of the SpeedTree Modeler (often 30 days), it comes bundled with a "Starter Library." This includes 10-15 fully functional tree models (like Pine, Maple, and Oak) that you can export for non-commercial or testing purposes. This is the safest and most official "free download" available.
If your search for a comes up short (legally), consider these low-cost or free alternatives: speedtree library free download