Ok I got a new kindle fire for christmas and now am going to give my second generation kindle to my niece.  It has very few kids books and I noticed that the books on amazon were quite expensive for her age group.  She is going to be nine and reads medium to large chapter books.  Recently she started reading the magic tree house books, box car children, little house on the prarie and some mystery books I can't seem to remember.  These books on amazon were a hefty chunk because they are series books and they were like 4-6 buck a book.  Does anyone have another online book store that I could purchase well named books like these for children at a cheaper price?  It is a kindle and uses the Mobi books but I have calibre and know how to change pdf's if they are open so that woudn't be a problem either.  Anyone who's got a solution for not spending a fortune on kids books would be appriciated.    

The Gutenberg Project has a lot of the older favorites. (Such as Robert Lewis Stevenson, Jules Verne, Louisa May Alcott, ect. ect.) Or you can get them at Manybooks.net
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At Amazon, in the Kindle Store, you can type in "Children" or 'Young Adult", chose by age (at the top, blue links) and then sort them by price, Low to High.

Here.

I've been looking for children's books also. My library system here in MO has a small number of them, free but you have to "wait" your turn to check them out and I wish there were more to offer. I also have found a few freebies on amazon.

http://www.amazon.com/s/ref=nb_sb_noss?url=node%3D4&field-keywords=kindle