A cut section of palm tree makes the best I've found (tough, fibrous, and practically indestructible), although it's awfully scarce, as far inland as I am...
However, if you can't find a section of tree trunk to turn on its side and throw at (throwing at the end grain is less likely to bounce your near-misses and will help your targets last MUCH longer), then one of the dense foam targets for archery is quite an admirable substitute. These are made to take broadheads at much higher velocities than you can throw. I had one forever, we used it inside, outside, and all over. The center, of an especially dense foam, is made to be replaced when it gets too chewed up (the targets run around $40 initially, with the center sections going for around $15-$20 each to replace).
While certainly not the most "traditional" of targets, it works, and they're usually big enough (mine was 3' square) to prevent you chewing up the surrounding wall, unless you're just WAY off...in which case you may need to re-think your style, anyway...!!
Good Luck!
Kal