If you're looking for retail (like, to buy shirts for yourself from) then Hot Topic sell a lot of music shirts,
some of which go to 3XL. Megadeth / Metallica / Evanescence seems to cut out at a 2XL (at the moment anyway), but they do have other similar bands (System of a Down, Slipknot, ICP, Misfits, etc. etc.) in 3XL if that's of interest.