Flax is also known as linen. Linen is considered a warm weather material and can be purchased at many franchises. Amazon would be one place. Wal-Mart, Target, Kholes are some to name a few.
Flax clothes are a specialty item that has not hit mass appeal with most department stores. The best place to locate clothing made from this natural fiber would be outdoor-oriented stores, such as R.E.I. or Dick's sporting goods stores.
There are many online stores specializing in flax clothing like helloflax, tender treasures, flax designes, funkyflax, flaxgirl, beach boutique, or nicheonline.