So much cool wrock merch comes, and then it's gone with the wind... Has there ever been something you've really wanted but stopped being made before you could get one?
I tried to think of something but I don't think I have. I pretty much always make sure to buy the merch I want right away. From personal experience, I know how expensive (and therefore difficult) it is to stock merch and even if something doesn't end up running out quickly, I've been there (heck, I am there) so it's worth the earlier purchase just to support the band.
My advice is, if there's something you want, get it the first time you have the opportunity. The band might not have it much longer and if they do, they might not be able to afford to keep playing shows where you could buy that item. If you can't make it to a show, you could also find out before they run out if you could order it online.