indeed. They are still shipped as part of the R510/R610/R7xx.
Part number below:
T-bar ceiling mount kit for
R720, R710, R610, R510, R310, R320, R700, R600, R500, M510 for mounting
to flush frame ceiling.
List price is 25$ which is quite steep... and lead time is usually long as its not an item usually in stock. Over on ebay, someone is selling a used R500 with a T-Bar bracket for 50 bucks. You might be better off with that and having a lab AP on the side.
If you need a lot of them though... I guess you might have to bite the bullet, unless you're willing to hack something up yourself.