I'm looking at https://docs.aws.amazon.com/general/latest/gr/aws-ip-ranges.html trying to work out what IP address ranges are used by AWS Lambda but in the linked JSON file I don't see any references to the Lambda service. Does AWS Lambda just use EC2 under the hood and are those the IP address ranges I should be looking at?
The only official answer I can find is on the official AWS forum (from 2015) is:
Unfortunately Lambda does not have a fixed set of IP addresses which it uses.
VPC support, which is in our roadmap, should allow you to control the public IP addresses in use by your function through the use of an EC2 NAT.
As far as I can tell, if you need to control/know the source IP of outgoing requests from your Lambda function, the official answer is still to put it in your VPC and use NAT.
Another idea would be to make a request in your non-VPC Lambda function and see what IP address you see. Then try to find it in the ip-ranges.json file and use the block of whatever service it turns out it is using currently. Just take into account that this may not work forever.
The IP addresses will vary. If you need a fixed IP on AWS for a Lambda function you can attach an "Elastic Network Interface". The Lambda function will then use this interface inside a VPC which can have a fixed IP-address.
I hope I was able to abstract your needs from the original question. More informations can be found here.
Hope that helps!