I don't know If I can say this or not, But there are some great seed sources on Listia, where the seeds are free, or postage only.
I don't know where some get their seed, because they have a lot of it, but other "seed sellers" are seed savers, and offer the seeds they saved. This site helps so many people, get non hybrid, and non GMO seeds, for very little cost, and supports ethical growers.
https://www.listia.com/search/home-garden-seeds-bulbs?location_id=111&q=
People bid with credits, not money.
It works just like a seed exchange, but with way more oversight and management tools.
I list my excess seed, for free shipping, and others do the same.
People bid on them with credits, not money.
When they win, you get the credits they bid with, then you use those credits, to bid on someone else's offering.
The only cost, is the cost of postage, to send out what people win from you. In turn, when you win something from a seller offering free shipping, THEY pay the shipping to send yours.
I won 15+ types of tomatoes, 3 + types, of squash, caraway, basil, cumin, Moringa, hardy kiwi vine, flowers, kale, milkweed, Aster, mulberry, sage, and a seed lot with 45 packs in it, plus shoes, clothes, toys, soap, and Christmas presents.
Credits can be used to bid on any item of interest, not just seeds. That's why it's better than a traditional seed exchange.
I gave away hundreds of packs of excess seeds, 20 to a pack, so thousands of seeds. They go quick, so they're very fresh for people. That's a hundred people that are going to grow next year because I listed excess seed.
If they save and share seed, that's a lot more readily available seed, being circulated, for little cost to growers, and no money in corporate pockets.