Thanks to everyone who offered suggestions and advice on audiobook cases and sleeves! 

Here are the number of recommendations by vendor (with the best links I could locate): 

Showcases = 7 

CCI Solutions = 1

Demco = 1

Polyline Corps = 1

Sunrise Packaging = 1  

Vernon Library Supplies = 1

WTS Media = 1

Additional advice included to be aware of shipping charges and minimum required orders. 

Again thanks, wonderful Michigan library community!