ZeroLemonOfficial via Amazon has battery cases for Galaxy S9 and Galaxy S9 Plus on promotion below.
$20 for Galaxy S9 4700mAh Slim Power Battery Case
Original Price: $29.99
Coupon Code: 5MFQ6OSO
$21.9 for Galaxy S9 Plus 5200mAh Slim Power Battery Case
Original Price: $29.99
Coupon Code: FS9QA56F