The "regular" military bands have openings periodically.
The Navy and Air Force has about 15 bands or so (some are overseas). The Navy has about 15 as well, again some are outside continental US. The Army has the most with around 30 bands. The marines I know nothing about, nor do I care to research them further.
These bands regularly have openings, simply because there are so many of them. Perhaps, for the sake of your project, you can simply have an entry for "military bands"?? When I was looking, I would have considered all these branches.
Symphony gigs and premier military bands do come up, but not often enough to be "reliable," often with several years of nothing before a job pops up. These bands have so many tubists (put together) that an opening does come up regularly. These branches have their differences yes, but not to any huge degree.