89 year old with dementia. Talks all night and day and is argumentive. What meds do I need to ask for just to get some sleep?

Take her to see her doctor. How advanced is her alzheimer's / dementia?

It may be trial and error for a bit. Some meds will work great for some and have the completely opposite effect on others. Ativan had completely opposite effect as intended on my mom. That was a nightmare. Yet, Ativan works great for many.

I use over the counter Melatonin. It comes 3, 5 and 10 mg. I use the 3 which I get at the dollar store. I do need to try the 10 sometime. Good luck.