9/16" hex shank chuck arbor?

Doublecheck that, Bob. I believe that you'll find that it is a 13mm hex drive, not 14.28mm / 9/16".
This is timely. I just ordered, received, and used a 12mm x 280mm hex shanked (13mm) masonry bit last week. Gunner had given me a bitless driver (Makita MA-2714) last October and I finally found a $12 import bit thru eBay. Uxcell and Amico have many sizes. Like you, I found -squat- in Home Depot, Lowes, or Medford Tool, or any local auto parts stores.
Who can afford one to see? Then again, they're designed to be used against a bit, not operated in space, so it may not shake as much when it's in use. Hammer drills do vibrate a lot, though. It's their nature. Anti-vibration gloves were designed AFTER hammer drills. ;)

If you have a lathe a better option may be to buy an A taper adapter for your tool.
Then you can turn a taper on the shank of any old bit to fit the adapter. I also have a rotary hammer that takes an unusual shank, an old AEG, but it came with an adapter so I grab used bits when I see them cheap. I don't remember off the top of my head what the angle of the taper is, but it's not awfully fussy and easy to turn with the lathe compound.

Just did. .562 inch across the flats and 2.25 inch long on an original Skil bit.
I bought this tool mostly to see what I could learn from it. Lesson number one is that vibration is a signficant issue, not only for comfort but also for tool control.
Lesson number two is perhaps more surprising: Searches for bits that fit the tool are incredibly laborious because the shank size gets confused with the bit size. If the shank was SDS or something equally arbitrary it'd be much easier.
Thanks for writing, I'll just have to keep looking.
bob prohaska

I'd be really surprised if the shank of a hammer bit could be turned in a lathe without annealing and re-hardening.
There do seem to exist hex-shank to SDS adapters:

Maybe I should look more closely at those. I really wanted to get away from proprietary toolholders. B&A sells very little in 9/16" hex drive apart from adapters to their own bits. It's possible to buy SDS bits at my local hardware store.
Thanks for writing,
bob prohaska

Maybe it'd be easier to adapt to other bit sizes

I've never tried tapering an SDS or similar shank, but I've turned a taper on many straight-shank bits, including a couple large core bits. None were hard enough to be a problem. Factory made A taper shanks are not harder than perhaps Rc30.

There ya go. That's the ticket.

At least in my neck of the woods, SDS seems to be the most widely-stocked hammer drill shank. I was hoping there might be a non-proprietary alternative, but that's simply not the case.
The problem now is to find an adapter that fits a Skil 732 rotohammer and takes an SDS tool. This might work: but the ad seems a bit "off". The review is nonsense, and somewhere in the fine print it says the item ships from the UK.
It seems likely that "native" bits with a 9/16" hex shank will couple better to the hammer, so I'll keep looking until there's some urgency.
Thanks to all for reading and replying!
bob prohaska