Hi Ryan!
Welcome to the Forum!
Sadly -- insofar as I have been able to determine -- Shimano does not offer or stock replacement bushings for this brake, spares being limited to replacement pads and pad holders only.
As with many similar items these days, the brake is considered a consumable item, to be used until it is worn out.
The good news is, one can overcome the worn bushings (and noise arising from them) with increased toe-in of the brake pads.
The noise is caused by the loose tolerances allowing the pads to dig in tail-first. They catch-and-release in a rapid cycle, and that is producing the sharp squeal you hear. Crank in some additional toe-in, and when the pads strike and catch the rim, they'll settle in flat but no more than that. It should largely solve your problem.
The parallel-push mechanism of these brakes is wonderful for reducing maintenance insofar as pad wear is concerned -- they'll hit vertically square pretty much throughout their service life. Sadly, it does nothing for toe-in and increasing pivot tolerances due to wear and use...the user has to monitor and address that themselves. Toe-in does the trick!
Hope this helps.
Best,
Dan.
