Cliffa, I don't know where the spindle end caps were sourced. When I acquired this bike, the PO had started work, then let sit for more than a decade.
In the few records that came with the bike, there is a reference to "add Rokr Spndl 'Lock' Plates (w/Alloy Covers & Allen Screws)", but no indication of where they came from.
He made several performance oriented mods, including the head steady & rocker spindle caps, but also Amal Mk 2 carbs, Koni Shocks and tapered roller steering head bearings. Those last two I am not using.