We may not usually condone the capricious waste of Microsoft Points on Avatar frippery, but we must admit that the Sonic & Sega All-Stars clothing set makes a pretty good argument for the existence of Avatar clothes. And that argument is "here's Ryo Hazuki's jacket." It's available for men and women for 160 ($2).
The other available items strike similarly at the heart of Sega nerds, including Beat's sunglasses, a little AiAi-in-Monkey-Ball prop, and a Samba De Amigo sombrero! You can even buy a Crabmeat to decorate your virtual Green Hill Zone (by which we mean your dashboard). See the full set at Xbox Live Marketplace.