Two words; boot boxes.
If you are like me, and have an addiction to shoes and boots, you'll probably have at least one box from the latter lying around the house. If not, ask your local shoe shop if they have any they are about to dispose of from customers not wanting to carry out their boxes.
I got lucky, by the fact that my last job prior to this one was at a shoe boutique.
Many times customers didn't want to carry the larger boxes after purchase, so they would leave them with me. Most of them I had to recycle, but I would always keep the really sturdy ones. If we had a bounty of those, I would take home at least one or two.
The nice thing about storing them in a box like this, is it allows the feather to lie flat.
For super long plumes over 32" (rare but they do exist), if a boot box isn't quite long enough, a mailer tube may work. My only issue with using these, is they compress the plumes more than I like, so I only use them for 3 plumes max. Stuffing a tube or even a box too full, might cause some crimping on the hurl (The hurl is the soft floaty tendrils of the feather, not what an ostrich might do after eating too much...as funny as it sounds ;-)
Just posting this image of an ostrich plume lampshade...because it's amazing.
One other trick I learned that isn't really related to storage; How to make new ostrich plumes do 'The Dip'. It's that nice bowing of the plume head that we see on vintage ostrich feathers, like the ones of mine in the photo on this post.
Curling the feather is one option for 'skinny' plumes that don't have much body. We see these kinds a lot at our local craft or fabric stores, or online. The quality is usually not as luxurious. You can , however; layer the skinnier ones together and wrap them with thin wire along the spines. I've seen this technique on many Victorian feather hat trims.
But, if you get the chance to hand-pick your plumes, get the ones with the fullest hurl and heads as possible.
Whenever I buy ostrich feathers that start off really straight with little curve, the first thing I do when I get them home is stick them in a slim, medium height flower vase. Leaving plenty of room for the length of the feather to remain free. This is only if you have patience, as it may take a few days to a couple of weeks, depending on how long and full your feathers are. Also, the softer the spine toward the last 6 inches of the plume, the better.
For really stubborn ones, a little steam may help too.