Walking home from work tonight at midnight (it's tough to be Batman when I spend so much of my time doing real people stuff) I found a boon. Someone is doing renovations to an old apartment building and there were a few pieces of scrap wood lying against the fence. It was clearly no good to anyone else but the blocks are quite thick (maybe 4' in width and depth) and maybe only 2"6' long. They might work perfectly as a target for the shuriken I should be receiving in the mail any day now.
Now all I have to do is find a hammer... I bet the real Bruce Wayne doesn't have to ask his neighbors if he can borrow a hammer.