White Labs Black Men's T (Microscope on the side) Part # GSTMICROBLK S-M-L-XL-XXL. The words on the back of these shirts say: "Advancing Fermentation, Cultivating Community".
Also available in blue and green. Green is shown below.
Order all Gift Shop items at www.yeastman.com, White Labs' ordering and tracking system. You must create an account first. Afterwards click the "Other Merchandise" tab, and scroll to the bottom.