I am trying to use Google Workflows to make HTTP POST requests to a service that uses a whitelisted IP list. How can I find an IP, or range of IPs that I could give to the vendor?
You can whitelist all the IP range reserved by Google Cloud. But at the end, it's like if you allow all users. Indeed, any users (or attackers) that use Google Cloud services will use one of the Google IP ranges, anyone will be able to access your service.
The best solution is to use a Cloud Functions or a Cloud Run as proxy. Cloud Workflows call that proxy internally at Google Cloud. Then, on the proxy service, you can plug a serverless VPC connect (with egress param set to all traffic) and a CLoud NAT to reserve a static public IP ONLY FOR YOU, and you will be able to allowlist it securely because ONLY YOU will be able to use it.
Here the doc of Cloud Run, but it's pretty similar on Cloud Functions.