name.com and namecheap.com ... search for their coupons online ... you can save a couple of dollars per domain.
You can sometimes find offers like 1and1 @99c each, but you will pay much much more in the long run, when it comes to renewing the domain name, or transferring it out.
Also, try and avoid GoDaddy too. They are an all round shit company.
While we are at it, and this is going to be controversial ... If you can't afford to be spending $10 or so per domain, then you are not ready for IM. Or you are wanting far too many domains.
I used to collect domains. Each one had a business I definitely was going to set up on it. Never happened. And I ended up just paying hundreds of dollars for domains I never used, and eventually let expire.
Now, I build the website first, and only when it is ready to launch, do I register the domain.
Saves me a whole load of money I could have spent on domains.
YMMV of course.