Assuming a 64-256K board, they are 4164 RAM chips. As far as I'm aware, they haven't been manufactured for a very long time.
They are certainly still lots of them around though in a salvageable (sometimes NOS) state for relatively low cost. They were used in lots of different machines. They do however seem to have quite a high failure rate so unless buying some that are sold as tested & guaranteed I would buy more than you need to allow for duds.