How do you get an onyx chao for Sonic adventure battle 2

To get an Onyx Chao in Sonic Adventure 2: Battle is to have Sonic Advance 1 or 2 (Tiny Chao Garden) and a GBA link cable to transfer it to the Gamecube. It costs 20,000 rings and it's a little rare. But it can be found at the shop in the Tiny Chao Garden. Hope this helps!