Generally I prefer to buy print books rather than ebooks. However, I own a NOOK ebook reader and I do buy ebooks, especially when I can find them at a low price. For discounted ebooks I use BookBub.
BookBub finds heavily discounted ebooks, audiobooks, and even print books. While I buy all my ebooks from Barnes & Noble, BookBub links to ebooks from all major retailers: Amazon, Apple, B&N, Google, and Kobo. In the BookBub settings, you can choose the formats you want and also the retailer(s).
I have well over 150 ebooks on my NOOK and probably 90% of them were purchased as discounted books via BookBub. Prices for ebooks are typically $0.99 to $2.99 with the occasional free offer. The books are not purchased from BookBub, but from the actual retailers. I estimate I buy one to three ebooks each month via BookBub.
You don’t need to visit the website to find ebook offers. Depending on your choice, BookBub will email you once a day or once a week. I get daily emails that include between six and twelve ebook deals. In the settings, you choose which categories you are interested in from many fiction and nonfiction genres.
I have no affiliation with BookBub, I just like the service and highly recommend it for filling your e-reader with ebooks at a low price.