White lily seeds are a type of seed used in the Farming skill which requires level 58 Farming to plant in a flower patch. When fully grown, white lilies protect all neighbouring allotments from disease.
They can be obtained as a drop from the Hespori, a Sporadic boss which takes up to 32 hours to grow after planting a Hespori seed, or bought from other players.
White lilies themselves cannot be protected from disease as gardeners will not watch over them as they grow.
For an exhaustive list of all known sources for this item, see here