If the words "respacing" and "new axle" are chilling for you, you can get them from IRO for sure. Dunno as hubsets or loose hubs though.
I just bought a complete fixed/fixed 26" complete wheel with an Aeroheat from IRO. I tried to go Kogswell and he referred me to Tony at IRO - they're out of them, or were in November. Cost me $110 shipped to NJ in two days.
I don't know if that means the IRO 135mm fixed hubs are identical to the Kogswells, are Kogswells, or Matthew was just plugging IRO, but they look very similar to the 120 hubs - burly locknuts and nice track nuts. The flange finish isn't as nice and there's no logo on the hub, but that's a moot point for me.
I haven't ridden it yet as I can't get the damn tensioner off the bike it's going to go on, and my usual shop is closed until next week.