The only sources is admin mode and the shadow ruins. The latter having very small amounts and requires you to destroy a nice looking ruin. Fix suggestion: There is these dark pillars (trees) found in the shadow biome which currently drops wood and plant fiber (no sapling). And these destructible black rocks that drops nothing. Have these drop shadow rock instead. Extra plus: If you have these (tree) pillars and rocks regrow back on the obsidian ground. In case someone wants to build a larger shadow themed keep without having to resort to admin mode.