I’m going to give you a specific answer in a minute, but first, let me point you to one of the finest resources for any card or board gamer out there, the card sleeve size list on BGG:
https://boardgamegeek.com/geeklist/1...ve-sizes-games
And here’s the Munchkin entry:
https://boardgamegeek.com/geeklist/1...28#item2912928
So, basically anything on that list will work.