Top mount? Do you mean the upper triple tree? If so I've yet to find one with the proper offset, spacing, diameters, heights and angles to just swap. I doubt it exists. I'm assuming you want one without the ignition mount and possibly bar clamp built in? I would say it will be easier to have one made than to find one that just fits. Any decent machinist can make a triple clamp (hell, I can make one and I'm a crappy machinist), just bring them the old one and tell them what you want different.
If you're trying to do this on the cheap you could spend the rest of your life looking for a tree that fits. It would be easier to swap the whole fork.
"To argue with a person who has renounced reason is like administering medicine to the dead" Thomas Paine
1985 Kawasaki Vulcan 750
2009 Kawasaki KLR650
2005 Audi allroad
1989 Jeep YJ