There are many different types of cards sleeves that work for Wingspan cards. Wingspan cards are 57x87mm, so they need a sleeve that accommodates these dimensions. Your choice will ultimately boil down to personal preference.
We use Fantasy Flight "Greens" (FFSo3) Standard American Board Game Sleeves to sleeve our Wingspan Cards. They are 57x89mm. The extra length of these sleeves is a little annoying and can be a slight nuisance when placing your cards on the player mat, but they are very durable and feel great in my hands. These sleeves are currently out of print and are becoming more difficult to track down.
Other people have reported that they like Mayday Chimera "Orange" sleeves which are 57.5 x 89mm, although there are some reports of slight size inconsistencies sleeve to sleeve.