its impossible there is a way through sepwich.com but the link never works so forget it
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From http://mystic.sepwich.com/srb2wad/launcher.html
You get it as a reward for collecting all the Chaos Emeralds.Collect 50 rings and either double jump (2.1.25 or older) or jump and press the spin key (2.2.0 and up) depending on the version you have.
Go to www.srb2.org/mb and search for "SRB2 Riders 1.46.3 link".
Hey you can go on 4shared they have almost everything like Srb2 wads,Srb2 JTE full, and Sonic stuff.
Go to minecraft.net and download the official game launcher. It does require a paid account.