Funny shape, that- bit bigger than a playing card so it won’t fit in most of hte standard collectors stuff… Might fit in the white cardboard boxes though, I remember them being a bit taller and slacker than they needed to be.
This would do it if you can find any in stock
https://www.amazon.co.uk/Ultra-Pro-6-Pocket-Platinum-Pages/dp/B00B5HJ6MA
Or you could use the standard 9-card sheets and just use the bottom 2 rows? Bit crap tbh.
There’s a kelloggs cereal box which is pretty much exactly the right width for a standard playing card, just cut it down to height and make a flap lid. I must have about a million old trading cards stored away in these things.