News from the Parco Archeological del Colosseo website:
https://parcocolosseo.it/en/2023/05/ticket-office-opening-colosseum-square/
It looks to be located on Via dei Verbiti, on the west side of the Colosseum.
They note that these tickets are for individual visitors only - meaning not for businesses - and while supplies last. They do not say if purchasing more than JUST general-entry tickets are possible so it's unclear if tickets that include the arena floor or underground will be available. They also don't note yet what time the office opens in the morning but my guess is that it's same as the park's attractions - 9.00 am - although I'd plan on lining up earlier if needing tickets.
Would love to see some firsthand feedback/more info from anyone who purchases tickets here!
General-entry Colosseum tickets sold online are timed-entry, and also include entry to the Forum and Palatine; guessing the same is true here.