Sega Shop selections brings you these 3 tees with a Tails theme.
The first 2 are variants of each other, one 'navy' the other gray with the same design. (Note that the navy is so dark it's nearly black) The design here is classic Sonic & Tails standing back to back. The Sonic is likely neo-classic, while the Tails seems more likely to be an older Japanese version of the art. Even if it is mix n match it still looks nice

because both are on model and nicely classic. They're done with blue outline (even Tails) and a bit of a 'drippy graffiti' effect on the blue/orange stars that surround them, and along some edges/the feet. It says "Sonic & Tails" at the top in a slightly graffiti inspired font.
The last shirt is a solo-Tails tee (still uncommon) with Sonic Adventure era art of Tails holding one of his tails. This shirt has been made before (so this is a variant) but that one was black. All of these are about 26 dollars each, in adult sizes and are Sega Shop exclusives.