As far as I know you can't buy them anywhere. As far as when or if there'll be an Anisong World Matsuri next year you should check up on Daisuki since it looks like they sponsored(?) it and was the page you'd go for any info on the event.
Also if you're interested in the penlight simply because it was something that impressed you for not being a cheap glowstick then you should look into King Blade penlights since they're basically the same and it's nice since they have no print on it so you could use it for other events and concerts too. They have some in stock in Tokyo Otaku Mode
If you want a folding stool, the telescopic collapsible version can go through security easily because a standard camping-type chair has exposed metal rods that can make security think the chair as a possible weapon.
https://www.amazon.com/Retractable-Folding-Portable-Camping-Activities/dp/B08C2D57ZW
Looking for a chair policy as well.
My experience from the last time going, we had something like the one you linked bought cheap from Daiso and they wouldn't allow it for being metal. We tossed it at security.
This time around I was thinking of bringing those retractable chairs made of plastic: https://www.amazon.com/dp/B08C2D57ZW?psc=1&ref=ppx\_yo2ov\_dt\_b\_product\_details
If you don't care about getting the official merch ones, you can buy lightsticks of the same brand on Amazon (or from link2110 below): https://www.amazon.com/gp/product/B01JFOY9Q2/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1