I don’t know if anyone cares about OS cards here but with the deep research I had to do to find an adequate solution I thought I’d share my findings on the off chance it’s helpful to anyone.
A lot of people on Reddit seem to be convinced the Pokémon jumbo cards do not conform to a standard and a commonly used solution is an Amazon product meant for scrapbooking that is quite a bit too large.
The Pokémon jumbo cards do fit an ISO standard, it just so happens to be an incredibly unsupported one; C5.
As far as I’ve been able to dig up, there is only one product on the whole planet suited for C5, these Prophila kobra sleeves made in Germany. And with the outlet’s listing being only in German it makes sense nobody would know about these.
I have some on the way and will update with a photo of a (hopefully) snugly sleeved jumbo when they arrive.
That’s an awesome find! Let us know how it turns out.
This comes up from time to time and I think a good solution was found a while back.
www.elitefourum.com/t/snug-fitting-penny-sleeves-for-jumbos/15273/1
Whatever it was that I ordered I stocked up on heavily and haven’t had to reorder since. I can take a look tonight as it seems @skinst 's photos are no longer working.
Gah, my nemesis: dimensions in inches, no wonder these didn’t come up. That said, they seem a bit on the tight side. If the ones I get are way off I’ll try them though thanks
Well they do work lol. XD; I dont have the old photos, but here’s a vending sheet (which is roughly the same size as one) in the toploader and sleeve:
Or you have the option to PSA slab it too lol
I personally use 6x9 BCW Photograph Sleeves with Ultra pro 6x9 Toploaders for my Jumbo collection. I believe they are the same as shown in the picture of @skinst above, since she was the one who recommended them to me a couple of years ago. 
Greetz,
Quuador
Any update?