Just Google buy used domain names or something like that.
BTW, if you are doing this for your link campaign, you would want to be sure to buy a domain name that is keyword related to your site. Google is beginning to crack down on things like that so you could end up hurting yourself more than helping.
Thank you zeekstern, I'm absolutely agree with you.
I've already purchased a 1 year old domain with PR3 - Tech related. It was an expired one.
By the way, when you are buying an expired domain, you can check it's relevancy in Webarchive (http://web.archive.org/)- it showes how a page looked like in the past. So you can see the content a page had before expiring.
I don't really know how my new Tech site will behave, but I hope Google will like it ;)