Jelitto, based in Germany but has a U.S. office. I only buy the smaller packet home gardener seeds, but everything has germinated and I'm pretty happy with them, You can see from the copyright on photos that Swallowtail resells a lot of their seed and I believe it's where Annies Annuals gets some of theirs (based on my not being able to find seed for certain cultivars anywhere else). A note with my order told me it had been hung up in U.S. customs for a few weeks. Shipping is not fast. But if you plan and have time, they're great. Price in euros, so watch the exchange rate. Also, some of the hybrid seed is VERY pricey (like $18 a small packet). I stuck to things about $3 per packet. Most there is very reasonable, just pay attention to pricing.
I expect a lot of folks will answer you with seed SUPPLIERS - some have trial gardens but it doesn't mean they produce the seed. Stokes Seed is a distributor (not producer), mainly to the trade. But, if you're more looking for commercial quantities of seed than producers of seed, you might want to also check them out.