Don't know what an XRF8 is, but all the old SA hybs are whitworth(less) threads which you can realistically only get from a bike shop.
Just take the nut from the other side of the axle to a shop and see id they have it.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1