I bought them from a german store called grundstoff.net. Continental clothing produces blank t-shirts and supplies mostly screenprinters. I bet I am vastly overpaying for them and you could get some sweet deals on bulk orders, but at 13€ a shirt I didn't bother to search for other stockists. For comparison, those american apparel sheer jersey loose crew summer t-shirts run 22€ a shirt over here and I haven't seen them go on sale.
I would search for websites selling blank and/or fairtrade t-shirts and go from there. Moreover the store mentioned above only got them about two weeks ago and cc updated their website around the same time, so they are fairly new to begin with. Might take some time till other shops also stock them.
Sorry, guess that wasn't really helpfull at all.