www.triffidnurseries.co.uk Has seed stock for most species of carnivorous plants. If you check out their Pinguicula seed section you should find a couple of US based species (P. Planifolia for example).
You can always check below for more stores that may offer the seed you're looking for. Because seed orders don't require permits and certificates to ship, you should be able to order from most USA stores as well (which should help you in your search). Do make sure any seeds you buy are legal in your county though. I don't foresee any problems mind you. the-list-of-legitimate-carnivorous-plant-suppliers-t8732.html