I have found sources for the fancy cultivars but not the original white daisy looking bellis perennis. I would like to find seeds or plants or both.
We don't make vendor recommendations but a quick Internet search did come up with some sellers, but none of the well known seed companies. So there may be some risk in ordering it online. They are considered weedy and hard to eradicate if you tire of them, so you might consider something similar but less invasive. Here is more:
https://www.gardeningknowhow.com/ornamental/flowers/english-daisies/caring-for-english-daisies.htm