Do you need to replace the whole hub or just the axle/cones ?
Or is it a complete Coaster Hub kit you need?
Skyway axle rebuild kits are around £15 from Alan's or Vert-Ego.
Sometimes you see alloy hubs on ebay - usually someone's drilled them out of an old wheel, rather than original spares.
Haven't seen a Coaster kit for a while, but you could always sacrifice an old wheel if you can get one cheap.