Finding this was no garden tour
After a long, ridiculous search that made my head hurt real bad, I made a discovery that I hope will save many people from similar experiences: the nice people at Swem Library at the College of William and Mary have put Jefferson's "Notes on a Tour of English Gardens" up on their website. In full color! (Warning: massive files!)
In the joint online catalog for William and Mary and Colonial Williamsburg, the record for this item identifies it as the work of the "Jefferson Project" at Swem Library. What is this thing called the Jefferson Project? I must know! Alas, my brain has had enough for today - that's a mystery for another time.