If these guys don't have a size that fits your needs, then it probably doesn't exist: Postcard Supplies - The2Buds Collection Protection Shop - Comic Trading Card Vinyl Record Supplies

I still support Bags Unlimited and their Large magazine bags:

(sppmaglr-100) -         LARGE\Movie Magazine Sleeve - $10.45

I like the self closing strip and the price.

throwi

For zines, I usually place them edge-first into a magazine-size bag, then wrap the bag right around the item.

I find the smaller size, and often the covers are a thicker cardstock, gives enough stiffness so that a board is unnecessary.  Plus they're stored upright on a bookshelf packed tightly enough that they're not going anywhere.

For digest-size rpg books, I usually place the in a magazine-size bag/board so the fit in with the rest of the collection.

Zenfinite wrote in Bags for 'Zines?:The books will be weak at the binding from sitting open forever like that. Books are supposed to be closed, so, it's very hard to show it being weak while it's closed. I just wrap the excess around the book, personally. Don't damage your books by stretching them open inside a plastic bag, please.

Well, I'm not sure you and I are talking about the same thing.  For zines, there is no "binding", really--just 2 or 3 staples.  It doesn't stretch them to open them because they're only, like, 8 or maybe 12 sheets of paper.  I'm not talking about saddle-stitched or even perfect bound anything.  Just zines made from 8.5 x 11 paper folded in half and stapled.  Do you still think that being unfolded for long periods would weaken the binding?

It depends how old they are.  If they're years/decades old and have been folded that long, then changing that is a poor idea.  If it's something that came out last month, it won't matter that much.