Where do you all get digital books?

I’d like to drastically increase my intake of native material in order to get out of the slump I am in + contextualize all of the wani Kani drills a little more. I know about 小説家になろう and 青空, but I was wondering if there is an easy way to buy or download contemporary, currently running books/book series on iOS without having to do complicated loopholes like buying gift cards or using VPN

Id also love some book recommendations for upper-beginner to intermediate level!

Thanks!