They're not really disappearing, there just wasn't that many to start with. The two big players are Pitbull and Interco for the larger tires (35"+). For smaller street stuff, there are some other manufactures as well. And there's a few retread shops, and gobs of .mil take off still coming in as well.
Whilre running a 16.5" limits your selection, its far from obsolete at this point. If anything there's actually a few more places that make 16.5" offroad tires than there were in the early 2000's. But its a drop in the bucket compared to the 15/17" rim sizes.
just get some treadwrights

idk. i'm looking into the mil tires b/c i have that parts donor explorer that's a frame/body waiting for a v8 and tons. would like to make it run some cheap 37's as it'll be a pizza and beer budget build. (later on.. when i'm out of college)