Erigeron Karvinskianus tend to be sold in most garden centres. You could buy a larger pot and divide it into smaller sections. I would not bother with places like Thompson & Morgan. You'll most likely get teeny bits that need growing-on in another pot before it can be garden ready. To me, that is just more unnecessary work.
Agree with @Borderline those postal system sized plants are too much hassle for the average gardener. Find a proper nursery that you can visit when they reopen or one that does mail order from the RHS database: https://www.rhs.org.uk/Plants/Nurseries-Search-Result?query=32487 Seed is also another option.
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Find a proper nursery that you can visit when they reopen or one that does mail order from the RHS database: https://www.rhs.org.uk/Plants/Nurseries-Search-Result?query=32487
Seed is also another option.
https://www.dtbrownseeds.co.uk/s.nl?search=Erigeron
If you want just a few then I would buy from Leesa’s Alpines
https://leesasalpines.co.uk/product-tag/erigeron/
Thompson and Morgan have a poor reputation and are best avoided.