Here are a couple of Kobo coupons. The 20%-off coupon should be valid through the end of December, and the $1-off coupon is good until the end of January.
20% Off Kobo eBooks with code "June20off"
Get $1 Off select Kobo eBooks - Kobodollaroff
To use the coupons, you need to enter the appropriate code during the checkout process where it says: "Have a gift card or promo code? Use it here." (If you've already used these coupons, they probably won't work again. But for those who haven't tried them yet, it's a pretty good deal!)