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There any good source of bare human arms other than the plastic Catachins and their (IMHO) poorly done sculpts?

The ones in the new Catachan command squad and the ones in the Catachan heavy weapon squads are really nice. About half the thickness of the Catachan troop squad's arms.
Other than that, I might look into the Elves from Fantasy. Some might have bare arms, but I don't really follow Fantasyhammer.

Flagellants and chaos marauders have bare arms, but they're pretty thick too; Wood elves would seem your best bet tbh.