Threadless (www.threadless.com) are a US based online T-Shirt seller where all shirts are designed by invidual submitters. They have good sales on every now and then, almost always posted on OzBargain. A good sale price is around US$9 per shirt, and sometimes they even offer free international shipping.
Threadless don't advertise shipping rates, but playing with the cart reveals these shipping rates to Australia:
1-2 = $9 3-5 = $15 6-9 = $21 10+ = $27