Rino used to list them but I haven't been able to find their website recently so they may have ceased trading. Very expensive. Might be worth looking around suppliers of polygon or splined shafting and bushes as such are functionally similar so there probably is market overlap.
Another vote for fabrication using the mill a trough method. I've done such in steel with the closing piece made same width as the trough and welded. Closing piece clamped down on a HSS tool blank to ensure that the hole is really square. Obviously closer needs to stand a bit proud of the sides to give something to weld. Closing piece only needs to go a very short distance into the trough, just enough to ensure alignment, so its quite practical to turn the outside round after welding. Done carefully you can't see the join. Tool blank usually needs tapping out due to post weld contraction and a few strokes with a file are required to get a nice sliding but shake free fit.
I stick welding a good inverter welder makes for a much easier job compared to the usual hobby rated buzz box.
Clive.