Looks like you can buy it bulk, either silica gel or clay desiccant, on Amazon (and a bunch of other places) as well as little muslin bags to put it in.
If you're looking for something more "natural" get the clay. If you're looking for something less toxic, you can try this stuff.
I'm now looking because while I'm nowhere near your league, Mob, I have to say, "Hi, I'm Andrew, and I have a nib problem." I just calculated and with purchases from the last few days I'm up to almost 1000 nibs. Two months ago I had less than 15. I think I'm done now. I only have about 25-30 different kinds of nibs, but they range across the gamut from stiff, to stub, to ultra-flexy. But what deals they were! You just have to look.
Now I need some of those holders you have with the cork grips. More of the ink! and the PAPER!! AND ...
Storage. Thanks for this thread.
And along that line, for those not in boxes, I saw someone on another forum using these little battery storage cases. They laid out the nibs horizontally where the battery would go. They fit a fair number of nibs in one case, you can label the outside easy, they stack and are cheap.
I happen to have a ton of small metal tins. I'll probably use those for the loose ones.
My experience with steel woodworking tools says to keep them in a cool, well-ventilated and dry place. No attics, garages or basements. If you're comfortable there, they they will be too. (I'm not comfortable sealed in a plastic bag so I can't imagine my nibs would be either.)
Good luck with your collection! If you come up with something cool let us know.
Now I've got to get back to using them all up. I have a lot of letters to write!