Zara has some on great sale for $9.99. They are decent-weight organic cotton and good quality, at least as well-made as the one I bought from Boden recently at $28 (which was discounted).

http://www.zara.com/us/en/woma.....03004.html

Here's a similar striped t:

http://www.zara.com/us/en/woma.....50659.html

I can't find the color I bought on the site so the store may have more options.