Skeletal boots are a member's only piece of Fremennik armour that are primarily worn by mages. Contrary to popular belief this item does not require the completion of Fremennik Trials, 40 defence, or 40 magic to wear. To obtain skeletal boots, either receive them as a Wallasalki drop or trade with another player.
Dropping monsters
For an exhaustive list of all known sources for this item, see here.