Answer: I just answered this question for someone else yesterday! Unfortunately, I've been unable to find a sourc for seed of Phlox subulata. It's not so easy to start from seed, I undestand, so cuttings/bedding sets are easier to work with. It may also be the case that only breeders work with the seed, and wholesale suppliers only propagate with cuttings.
Apparently P. subulata will sefl-sow, but the flower colors aren't reliably uniform.
To cover a large area, you can divide the plants you already have and spread them through your garden bed. Autumn is the best time to divide phlox in the south, but you can make cuttings of the plants in spring. I hope this helps!
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