Well, I don't think that it is available on the web; I looked, and had no luck. I actually have a copy of the poem, because I own the book "New and Selected Poems: Volume One" by Mary Oliver, and the poem is in there. This book was published by Beacon Press in Boston, in 1992, on pages 201-202, just so you have the information for a full citation for it. Here are the first few lines (the back-slashes indicate a line break):
"My mother and I debate:/we could sell the black walnut tree/to the lumberman,/and pay off the mortgage."
I hope that helps with your homework a bit; it's a great poem about the powerful memories and familial ties that trees and other great structures built by strong hands can have, and a tribute to respecting the work and cares of those that have gone before us. I hope that you enjoy it! I also provided a couple great links to some discussions of Mary Oliver's style and themes, which might help with your homework too.