phil wood, offhand, is probably the highest end brand that makes a hub with one side with stepped lockring threads and the other without (for singlespeed).
that said, however, you can use any double-sided track hub, because freewheels work just fine on track threads. they end up engaging with fewer threads than a purely single speed hub, but it's a non-issue. you also have the advantage of being able to do two fixed cogs, should you end up not wanting a freewheel at all. if you aren't doing a ton of bad weather riding, this opens up the possibility of using dura ace hubs, etc.