Mushroom Grass Seeds

Lay a wide bed of mud on the surface, plant, and wait. The spread is slow, roughly on par with jungle grass, so start it long before you actually want the vendor and check back over several in game days. He needs a house standing inside the finished biome, not beside it, and the patch has to be large enough for the game to register a biome at all. A narrow strip will grow mushrooms and never count.

Below the surface the same patch gets you a mushroom farm and a light source without any of the biome requirements. Giant mushrooms only sprout where there is headroom, so leave a few tiles of air above the mud if you want the tall ones for crafting rather than the small ground cover.

With a Discount Card and a Dryad kept happy, the seeds get cheap enough that shimmering them into jungle grass seeds, crafting those into thread and selling the thread back turns a healthy margin. Everything involved stacks to the same high limit, so the ceiling is however long you are willing to sit at a loom. At neutral prices it loses money, so do not attempt it until the discount is in place.