You can pre-order the Official Sonic X Animation Soundtrack at Yesasia.com now.
I highly recommend this site because it has a lot of Asian imports (hench the site's name-Yesasia) at very affordable prices (most imports i've noticed online are at least $45-50 at a minimum price), and because the shipping is very speedy, usually within 1-2 days if in stock.
Sonic X Animation OST Info Only $23.99 at Yesasia.com.
Better to pre-order now than have it sold out later. =P
Source: Yesasia.com