and of course there is always American Apparel, made in the USA by unionized workers.
beantownkids can buy it at the garment district (kendall square stop on the red line). i'm sure they also have a website.
i know from experience that their shirst are the most god-awful comfortable things on the planet. they're what i consider pricey for a tshirt (i'm cheap) at about 25 bucks ea. but its definatly worth it. i know that a number of bands on major record labels are using american apparel shirts for their band t-shirts while on tour. very cool.