I use a mix of Alder, Oak and Indian Almond leaves in a few of my tanks. So far I've run into no problems, although I've found that Oak leaves tend not to stain the water as much as the Indian Almond leaves. However, they do seem to last longer before breaking down, and they're free if you live near an Oak tree.
Because they take a while to sink, I usually brew up a tea (just pour boiling water over some leaves) and let it sit out overnight. They should sink fairly quickly (if not immediately) after being put into the tank, and you can use the tannin-rich water left behind during water changes. I just store mine in a jug in my wardrobe and my Wild bettas and Killifish love it.