Cloud providers have a great infrastructure to perform DDoS simulations. You can get thousands of virtual machines with different IP addresses, fully customizable with the tools you need for testing. But as far as my research is concerned, most of them do not allow DDoS testing and explicitly forbid it in their ToS. Amazon for example states:
"You are prohibited from utilizing any tools or services in a manner that perform Denial-of-Service (DoS) attacks, or simulations of such, to or from ANY AWS asset, yours or otherwise."
Is there an established cloud provider that allows DDoS simulation and can be recommended for DDoS testing?