Interesting timing. I happen to have spent the last 2 weeks trying to resolve this very issue for RCBS Safari dies. This bushing (at least from RCBS) is NOT readily available, and come to find out has been discontinued for quite some time. A whole week of working higher up the chain with RCBS and Vista Outdoors, and highlighting the fact they readily sell Safari Dies that are currently impossible to run in any RCBS press, I apparently created quite a stir. A rep contacted me directly and said she located some and could ship me one. She also informed that management has now been made well aware of this issue. In parallel, the previous day I located and ordered a bushing from another vendor, CH4D.
CH4D bushing is half the price. If you want RCBS, call their customer service number, get to RCBS, then ask for Robin.
I never got a straight answer on why this item was discontinued. Looking at a spec drawing, I do know that the differing thread pitches between 1 1/4" and 1" do not allow much "meat" at all between the two threads.