If you are planning on using your phone, I suggest getting an official Google Cardboard unit.

https://arvr.google.com/cardboard/get-cardboard/

There are plenty of other phone holder style units but they may not work with the cardboard app(Which has a large amount demos/apps)

If you are looking for something more advanced and can wait a bit, I would do that. I would expect the console gamming systems will be pushing out some new offerings soon.

Occulus is an option but from my understanding it is married to FaceBook/Meta and that is a risky proposition IMHO.