They are a Thorn product, nobody other than SJS sells them that I know of. I have a couple of them, but I ordered a bike frame and fork and lots of other stuff from SJS so the shipping cost to USA on a per item basis was not too bad. If you only want one item like that, you can buy it from SJS but the shipping cost is very high to USA.

The Thorn ones are strong metal, not the cheap light plastic ones that some use for computer brackets, the Thorn ones are designed for the weight of a handlebar bag.

Any reason you do not want to use a second stem? I know that you want to put it on a seatpost, not fork steerer like in my photo, but I would think that with the right shim you could mount a second stem on a seatpost. In the photo I use a short section of PVC pipe in a second stem to hold the handlebar bag bracket, but a piece of cut off handle bar would be stronger than PVC pipe.



I have the 55mm length Thorn T bar in this photo on this bike.