The website Wired for Books has a wonderful selection of Beatrix Potter books translated into Japanese. The most familiar of her writings is of course Peter Rabbit.
Although these books do include some kanji, children's books typically are a great way to practice reading Japanese. It's hard to jump into reading manga in Japanese because the writing level is too high for beginners; you spend more time flipping through the dictionary, rather than the story.
Try starting the classic Peter Rabbit.