Great House (Morrowind) Furniture: Hlaalu, Telvanni, Redoran, Dres, and Indoril furniture plans have a chance to drop from any qualifying containers (or lock boxes) in the Vvardenfell zone.
Where to Farm: Dreloth Ancestral Tomb (south of Sadrith Mora) has 100+ lootable urns. If you start the related quest, but don’t complete it, you’ll have unlimited access to the entire tomb—otherwise, the second half of the tomb will be inaccessible.
Bonus: Morrowind Furniture Documents (sold by the Masterwork Vendor) are a reliable source of these plans. If you spend less than 1000 gold per voucher on both master writs and the materials used to complete them, you can generally guarantee a profit—and some plans are worth far more than the 50k-per-document you invest.