Steyr L9 threaded barrel, anyone know where to get one?

As far as I know, they have never been made. You will have two options - you can try a regular M9A1 threaded barrel - it MIGHT work (coincidentally, I have one for sale... ).

In the 8 or so years I've been heavily involved in the Steyr community, I haven't seen a single aftermarket barrel company that was willing to go near our guns - your only choice for a custom-made barrel would be IGB Austria. Communication with them can be problematic and they have a very sketchy history regarding delivery times, but the products seem to usually be on-par with the Steyr factory-made stuff.