Most Harley aftermarket sites have it. You can get it in two diameters and a few lengths. Some of the diameters are a little non-standard and there are no mounting bolts on it, so get it from a place that also sells the appropriate sized P-clamp to secure it. You may need a bit of a shim to bolt it to the pipe too.
My other fave is the Flash Gordon 40s Harley style. Again, not technically proper on a British bike but it still looks cool.
Scott