Hi, you cannot buy seeds they are protected, you can only get them from saved seeds from someone growing them or by buying the onions as you say.
Last year I was very lucky as I got a bit of seed from another poster . Need another year though for seed.
I then managed to get some onions sent over and grew them this year, I have seed from those.(Still have the label that came with them, it has a phone number on)
I have a little seed I can let you have if you can't find any anywhere else but it will only be a pinch.
I also have the Banana shallotts, again very little.
Pm me if I can help.
I tried for three years to find seed and was very grateful to get a start.
XX Jeannine