I've used both customink.com and customizo.com. They both have several options of 100% synthetic tech t-shirts.
From CustomInk, I ordered the "Champion Short Sleeve Performance Shirt"
http://www.customink.com/styles/champion-short-sleeve-performance-shirt/165600#image-link
And while the shirt was great, I was not impressed by the quality of the heat transferred image/logo. It looks like they used too much heat and you can see the clear part of the transfer material.
From Customizo I went with the Gildan Performance Tee.
http://www.customizo.com/catalog/product/view/id/57839/s/gildan-performance-tee/category/66/
I was pleased with both the shirt and the quality of the ink transfer. They were not as fast to ship as CustomInk but the price was better and the quality much better.