I have heard a vicious rumor that you can't get them from Triumph anymore (along with a lot of other accessories). That leaves making them yourself. I can send you the pattern for the Triumph ones through email if your interested. I made mine out of 1/4" plexi-glass.
Then I got a deal on a windshield fixing kit on eat-me-bay. Makes a nicer looking set up then the clamps that came with the Triumph lowers. Here's some pics of mine. They are a modified version of the Triumph lowers.