Sonic Team released it along with the normal Sonic Adventure 2 game on Sonic's birthday. You would have had to be right there to pick one up as they sold out extremely fast. You may be able to find one on E-bay, but I highly doubt anyone would sell one.
About a year ago I heard that Sonic Team were going to release a sonic compilation to celebrate his tenth birthday. But I haven't heard anything since so could you enlighten me,