Bethesda now has official Doom and Quake (their other brands too, but I care about the D&Q's!) T-Shirts for sale. You can get them for $18 in small to extra-extra large (for those extra-extra fans). The Doom shirts have the classic Doom logo on the front and the Quake shirts have the original Quake symbol on the front.
They're available at the Bethesda Store right now.