Can you suggest me methods or tools that can help in simulating a DDoS attack in a controlled enviroment (like my private lab)?
Since DDoS is not really a specific attack, on a specific vulnerability, testing for it is kinda pointless, especially since there's not much you can do about it anyway, except for scaling out more. (See my answer here for explanation why).
What you should be doing, is doing load testing - if you can handle 100K legitimate clients, you know thats how much you can scale against DDoS... and a DDoSer would need to scale up his botnet that much in order to succeed in attacking you. If you can't - well, then DDoS isn't really your issue, is it? (Of course, replace 100K with any number that makes sense for your site, given your business, risk, availability requirements, exposure, etc).
If you're just looking for a way to simulate that, the best way I can think of is some form of "cloud" based systems, e.g. throw up a large pile of virtualized machines on Amazon's systems and have them all go to your system. Of course, these would all come from similiar IPs, and DDoS come from all over - so maybe in addition to Amazon, you also want Azure, and also etc etc.....