I bought mine on Amazon. Remember the title in Japanese is "Animoufu" (in katakana, a-ni-mo---fu).
Japanese Animorphs on AmazonI bought them when I was in Japan, and the less expensive ones... Now they're getting expensive and you also have the shipping rate who increase like hell when it's sent from Japan to another country. The books themselves, as second hand, are about 200~500 yen each ($2~5), but you'll probably pay them 1000 yen more (each, except if you buy them from the same seller, maybe it'll be a little less expensive). Just as I'm looking, "Takaraya" sell 4 of them, and the last one is sold only 190 yen by somebody else. Also, sometimes people don't accept to send to another country...
So, good luck!
PS: If there are really problems about the shipping (nobody want to sell to America, for example), and you really want them, I can ask a Japanese friend to receive them and send them back to you, maybe. It'd take more time, of course.