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All sorts of good sources - SRGC , AGS seed exchanges, and the Hardy Plant Society, Cyclamen Society crocus group, Frit group, and so on. Then there are the commercial seed sellers, many of the best are shown in the pages of the forum when a new list is published... like Botany Ca, Kurt Vickery, Holubec - http://holubec.wbs.cz/ and many more. Try a forum search for Seedlists and see some great ideas. And forum members can offer their spare seed here as well of course.
You could type in where to buy + name of plants and your country or UK to see some named suppliers. I have used quite a few new plants using this method. Make sure the payment or the status bar displays a padlock so that your money is safe.
Agree with all of those. I would add Alplains (really good for N American); Michal Rejsek (wildflowerseeds.eu) (mostly European); Vlastimil Pilous (alpine-seeds.com) (also mostly European); Chinese Alpines (fairly self-explanatory!).If you are interested in Hellebores, I can also recommend Phedar Nursery (Will McLewin). Anne Wright (Dryad Nursery) does a very good Hepatica seedlist in the summer. Tristan