"Replika" was the app most of them were buzzing around last I checked. There was some boondoggle with paid plans, I think the app had tried to focus on that subcommunity as whales (singly focused big spenders).

Probably start there and see where they've gone now.

Not exactly AI girlfriend territory but has a lot of overlap with it (as well as a lot of overlap with mental illness and probably occasionally just plain old cringy teenagers) but r/waifuism is also pretty similar.