Hmmmm there are some chemicals in willow bark that might prove problematic... However, you often seen 'weeping willow' growing near to/on the banks of ponds/rivers...
In terms of 'wood' in general, pets at home currently have '3 for 2' on generic bogwood and Mopani.
I scouted forums/ebay etc. for mine and managed to get a good couple of pieces for £25 posted... there is also the option of 'making' your own artificial stuff...
Hope any of that helps! (there are a couple of good facebook based swap/sell groups that might help with fish related stuff)