Indoor or outdoor? They used to sell an indoor “Dome Siren” but I don’t see it for sale anymore. If you find it don’t bother purchasing it because it is a piece of junk. I got one and returned it because it wasn’t even as loud as my alarm clock.
They also used to make an outdoor siren but had to discontinue it because of a lawsuit brought by the alarm company ADP, a leading competitor. Ring chose a shape and color combo for their siren that was an exact match to ADP’s registered trademark ADT and Ring reach settlement in trademark dispute over octagonal blue signs - The Verge
For outdoors what I did was to buy one of the cheaper cameras that has a speaker in it and you can link it to your base station to sound the siren when the alarm is sounding as an additional speaker device. I posted some instructions here how to do it: (How to configure Spotlight Cam Plus / Pro to sound siren when base station alarms)
The camera speaker is not very loud either but I suppose is better than nothing and gives the added benefit that for around the same price as a siren you are also getting a camera.
While you are at it maybe you could vote for my enhancement request to make the indoor chime be able to be a siren at this post: (Please make Ring Chime a "Linked Device" option device). Ironically, the speaker output on it is much louder than both the “Dome Siren” and the Ring base station so it would make a good indoor siren.