I have several trees from Mr Maple and a few from Mendocino Maple and a few from forestfarm.com. All three provide excellent trees and great service.
I'm not that concerned about grafts, good or bad, they're all workable trees. Acquiring the cultivars I want to work with is my main concern, but if you mention that you want to bonsai when you order they will all select a better tree for you. Better meaning lower, cleaner grafts, but you still have to work the trees.