I have a broken air conditioner that plugs into my 220V USA wall
socket. Its prongs look quite different from typical 120V appliance
prongs. Before next summer I'd like to buy a 120V portable room air
conditioner. Is there a converter available that would allow me to
safely plug the 120V 3-prong socket portable air conditioner into the
220V outlet? Thanks.

Why not buy a 240v unit so you don't have to rework/run new wires?

I'm having trouble finding a 220v "portable" air conditioner that's
designed and priced for a bedroom-sized area. I was hoping there would
be a converter box available that could make my American 120v appliance
compatible with my American 220v socket.