Unfortunately, the full soundtrack was never released by itself. The only item for sale would be the single by masayoshi yamazaki. IIRC, one of my friends got the accompanying soundtrack when he bought the film dvd in the box-style limited edition. Seems that it would not be sold separately.
On the side note, you could try out places like amazon.co.jp, cdjapan or playasia (free shipping to sg) for a range of Japanese anime soundtracks.