They're really difficult to come across, to be honest your best trying to make your own.
If you have PSP, then I'd grab a good photograph with a practically mono backdrop (like a bright blue sky), then I'd mask it, by creating a mask layer based on luminance then inverting it. You'd end up with a kinda white tree on a black backdrop, which is far easier to fix than the original image. Once you tidy up the mask (maybe adjust it's contrast so it's bright white and erm, dark black) you can save out your results as alpha transparent images. You might have to copy the mask layer somewhere to edit it, so remember to put it back where it belongs.
It's tricky, and will probably need a bit of practice, but honestly if there was a good resource of this type of media then we'd know about it. The problem is exactly that it's so tricky to do - it's not like a simple texture that you can churn out in 3 minutes, it takes quite a bit of patience and care to get the results.
There is always that tree object generator (Treemagic?) though, worth considering because you could render and billboard it for distant tree's while retaining the object for close detail. This can be a god send if you want a lot of different tree's - billboarding can look quite tacky by todays standards.
