Just been sent the link to Modernist t-shirts, so thought it a good idea to pass said website on.
Based in Australia, it sells (as you might have guessed) t-shirts, some of which have a mod-friendly twist. Like the one above for example, which is a one-colour screenprint 'inspired by the 1960s album Ready Steady Go!' (their words, not mine – sure I've seen the image somewhere else, but the brain cells can't quite place it right now).
Anyway, check out that and the others online. The above sells for $45 (Australian) and shipping is worldwide.