I can confirm the Super 9 carbon clincher is track convertible and the track axle is readily available retail/new. It's the most expensive disc they make, though. I found one pretty heavily discounted on eBay that was in good shape. I'll be using it both on the track and for road TT's (going to sell my Jet+ Disc).
I've heard pretty good things about the Renn disc, but it's all 2nd/3rd hand information I haven't seen one in person.