It’s working today! Quick, create your instance before it stops working again 
other than that, I found this link to be excellent:
https://cloud.google.com/compute/docs/gpus/
For each GPU, it tells you which zones offer it. Thus, since https://course.fast.ai/start_gcp.html recommends using a P4, you can see that currently the zones equipped with it are
- us-west2-c
- us-west2-b
- us-central1-a
- us-central1-c
- us-east4-a
- us-east4-b
- us-east4-c
- northamerica-northeast1-a
- northamerica-northeast1-b
- northamerica-northeast1-c
- europe-west4-b
- europe-west4-c
- australia-southeast1-a
- australia-southeast1-b
- asia-southeast1-b
- asia-southeast1-c
Thus, when us-west2-b is not available (which is happening pretty often lately), you can start trying from us-west2-c and keeping moving further away geographically until you find a zone which has a P4 resource available. That’s what I do, and it works pretty fine for me.