Hi I have an A2 art folder with plastic sleeves which works for me for storing posters that size and below. But many posters I buy are slightly bigger than A2 and so stay in a poster tube which means that they aren't stored well and also I can't look at them. (UK based)
So how does everyone else store posters apart from framing them?
You can always store them flat in a "Planschrank" (Sry, google could not find a translation for this one).
Basically a cabinet with many flat but wide drawers that you can completely draw out.
My tip is to get a used one, as new ones tend to be expensive.
I guess they are commonly used by architects, printers and of course artists.
Typical english for these units is a "flat file" and yes, they do tend to start around $200US and can go well over $1000 if you're inclined to pay that much. Of course, scouring craigslist and being patient can net you one for much less if not getting it free (as mine was - right place at the right time).
That said, there are a few other options for pieces bigger than A2 (which is around 18"x24" in standard size, for those in the US):
Baroque Portfolios makes a line for larger size prints - these are geared specifically to poster collectors, but really most any art portfolio in a larger size will work fine. These can also be pricey (as in a couple hundred $US). Check some out here: https://www.baroqueportfolios.com
You can also go the "cheap and easy" way and pick up a couple sheets of acid-free foamcore (anything larger than the largest print you want to store in it) and make a "sandwich" between the foamcore and your poster, then clip the edges of the foamcore with binder clips or other hardware to keep the stack together and from shifting. While I would not recommend this long term - mostly because I know personally I can't rely on keeping a stack protected for a long period of time - I have known some collectors who have a number of prints store this way in multiple stacks on folding tables, under beds, etc. If you store multiple prints in a single sandwich, I would recommend spacing them with a sheet of acid free tissue or white butcher's paper between each print and as a buffer to the foamcore on the outside layer.
Lastly, you can also order sleeves of varying rigidity from places like bagsunlimited.com or bcwsupplies.com to sleeve your prints. Options here range from poly bags (kind of like a comic book or magazine bag, usually about 3 or 4 mil thickness plastic) to mylar sleeves (much more durable long term, but with some flex) to hard cases (think something comparable to hard cases used to protect high end sports card collectibles. All have their positive and negative aspects for storage, but all work to different degrees depending on what you're looking for.
Personally, I have about 8 of the 18x24 Itoya portfolios (along with a few smaller ones), a small 4 drawer file for handbills and miscellaneous stuff, two large four drawer files for the majority of my standard size posters, and then a table which is mostly for flattening but often has between 50-100 posters stacked on it. Most of these are in 4mil poly bags with some in mylar or hard cases, but some just "float" between paper in the big stack.
Great, thank you for the detailed reply. I'll let you know how I get on.
Just to update, I found an A1 (594mm x 841mm) / (23.4" x 33.1") zippable artcase which has a ring binder mechanism and holds I think around 20 or 25 sleeves.
As the posters have been in tubes for quite some time, they try and curl up when they are just too big for the A2 art folder, but hopefully will flatten out over time.
Next task is to take some photos and add to the database.