These days your at the mercy of the parts store vendors that have what your looking for.
KH parts go out of stock, more often than babies go thru diapers.
There just aren't rebuilt parts left anymore in the parts system, for sale.
Get your existing booster rebuilt with Booster Dewey.
As stated in reply #2, there are numerous parts vendors out there.
Your own research will turn up something.
If you might be interested, look in the parts for sale section.
I currently have a pair of rebuilt KH, 4 piston calipers, for sale.