Specific funding sources for rare plant conservation may vary greatly by region, taxon, and year. In the video link below, Mary Pfaffko from the Georgia Department of Natural Resources describes sources and strategies for securing rare plant conservation funding in Georgia, which may have parallels to other states and regions.
Nationally Available Rare Plant Funding Sources (List in Progress):
Section 6 Grants: Funding for the conservation of federally listed species from the US Fish and Wildlife Service and administered through States and Territories. Section 6 funds a variety of conservation activities including restoration, reintroduction, outreach, propagation, genetic studies, conservation planning, and land acquisition.
National Geographic Species Recovery Grants: These grants support priorities of IUCN Species Survival Commission Species Conservation Plans as well as conservation actions that are endorsed by the relevant IUCN SSC Species Specialist Group. Current plant groups with species conservation plans include: Cactus & Succulents, Cycads, Orchids, Palms, Conifers, Bryophytes, and Mediterranean Island Flora.
The Mohamed bin Zayed Species Conservation Fund: A fund that support individual species conservation efforts targeted to local and grassroots projects, but applications for larger grants will occasionally be considered at the invitation of the board. Typically prefers in-situ over ex-situ conservation.
National Fish and Wildlife Foundation: A variety of conservation programs with business plans specific to regions or project types.