The issue is 4" by 5-3/8"
So it should fit ultra pros (or any other brands) 4x6 sleeve perfectly.
On amazon right now for half off. But probably even cheaper from supplies sellers at shows.
https://a.co/d/7QQ9Hws
I'd probably double check with other collectors of the set that the fit isn't too snug. If it is you'd have to move up to 5x7 soft sleeves. And those won't fit in large semi rigids.