Where can I buy the comic books?

I like to read and collect comic books, but i dont have much money to spend on really expensive stuff.
Now I' m reading free online manga on Http://www.boxmanga.com .There many over 4000 mangas such as Naruto Death Note , Bleach ,etc. I would like to collect all these manga. Anyone knows where to buy these comic book . Where the prices arent really expensive?