I think the gnarled staves have only ever been available as loot =)
I'd been building a trellis too (just got it finished when I decided to move house =P) and I reckon the place I had most luck getting them was while killing wisps for peculiar seeds - not tht they drop a real lot of them, but it was a bonus for me when they did, cos I was out hunting them anyway, and they seemed to drop them more often than anything else I tried =)