You can find regular editons on CDs and DVDs at a normal price on yesasia or cdjapan, but like everything else, limited editions which are not find online are now quite rare (well it depends) and the price might go really high (like for exemple, on of the rarest Arashi CD edition is the 5x5 LE, and this autumn, I found it in Japan for 70000Yen...).
If you live in a big city, you can maybe find a Japanese second hand shop.
You'll find all you need to know in the Arashidex: http://arashi.fukanzen.net/group/articles/buyersguide/