Hello Les,
I wish I could help. I also use the Mk1 LGB Catenary, with the Stainless Steel loop ended wires too.
I have recently extended a catenary equipped branch line and looked into the possibility of finding more masts etc myself, although in the end I had JUST enough to wire my extension and loop!
I did however find that e-bay.de was a good place for this sort of stuff and it has the advantage of relatively cheap postage and no import charges.
The catalogue number for the plain arms was 6000/2. The only ones I can find on e-bay at present are United States which would cost a lot to ship plus the dreaded customs charge, but complete old style 6000 mast are available from Germany at less than £7 each. I would suggest that you keep looking there.
James