In my IM game, I cleared out all of the wood spirits from Trynton, but no one dropped a Spirit Staff. Where/when can I go to find random monsters that might give me one?

This is all based on the assumption that the Ebon Staff is worth having. Am I wrong here?

I don't think you will find many more tree-monsters. There are some in Ascensions peak though, but I guess it will be long before you go there. So my suggestion, if you really like the ebon staff, go for the flame staff in rapax rift, it's slightly better than the ebon staff.

Or the Staff o'Doom, you can sneak over to it if you're stealthy enough. (it's safer than Flame to get only if you go at night, when there's a lot less chance of meeting a buncha flamers)

You can fight Burz and let him summon his Woodland Spirits. They may drop a Spirit Staff. The Ebon Staff is a great weapon in mid-game because of its bonuses but there are better staves out there in the later stages of the game.