All you need to is enter a response status and response body and ResBin will generate unique API endpoint for you to use. Then you can make API requests to your new endpoint the same way you would any other HTTP request, and you'll receive the response you built!
ResBin is completely free to use, and also open source https://github.com/dasveloper/ResBin (repo is work in progress).
Currently response bins only last 90 days, but I'll likely remove that limitation soon.