volumes 1-3 <---not in the anime...well, most of it...volume 3's last chapter, I believe is in the anime.
volumes 4-12 <-- mainly follows the anime...I've heard there are a few differences though.
volumes 13-26(?) <---not in the anime...except for volume 13, some of it is in the anime.
so you will want vol. 1-3 and vol. 13-26.
I believe that answers your question...well, most of it.
EDIT:
About buying the manga...I bought from a fansubber that has an online site. He sells them for $6 per book and has a special where you can get vol. 1-20 for $100. All of his manga is in Chinese however, and not Japanese. He is very reliable. I stopped buying them though, because of the US release coming soon.
His site is http://www.aoshianime.net/
volumes 4-12 <-- mainly follows the anime...I've heard there are a few differences though.
volumes 13-26(?) <---not in the anime...except for volume 13, some of it is in the anime.
so you will want vol. 1-3 and vol. 13-26.
I believe that answers your question...well, most of it.EDIT:
About buying the manga...I bought from a fansubber that has an online site. He sells them for $6 per book and has a special where you can get vol. 1-20 for $100. All of his manga is in Chinese however, and not Japanese. He is very reliable. I stopped buying them though, because of the US release coming soon.
His site is http://www.aoshianime.net/