In addition to those mentioned by reidrothchild, you could have one custom made. I know that Proper Suit
is now doing unstructured jackets and I wrote about them today at my blog
and I took some photos of their fabric swatches.
In addition, Kent Wang is also offering unstructured jackets now, too, I believe. You can check his affiliate thread for more info.
FWIW, I have had jackets from LBM 1911, Boglioli and Caruso (for Burberry) in the past. I also know that Brooks Brothers has a few unstructured jackets, but they're cut quite short in the hem. The one navy cotton knit blazer I have is more like a cardigan in function than a blazer. If I'm not mistaken, J.Press even has a few options now, too.
(EDITED for grammar)