I managed to find a copy in the used games section at Media Play last week for $15.99. It was strange, I was gonna browse the PS2 games because I glanced at used cube games and saw about 8 copies of some Turok game and another 8 of some Scooby Doo game, but I had the strangest feeling to look for Ikaruga. So I look though about 3/4 of the used cube games, and bam, there it is!
Anyway, you'll just have to keep looking around, try Media Play like I did, but if all else fails it's only aournd $30 on ebay.