Eeekk This is a tough one.
First, domains are registered for a year at a time. To purchase a domain name a person goes to a domain register like GoDaddy.com, types in the name they want, if it is avaliable.. you can purchase it for around $10/year. Now if you search for a name and it is already taken, there is a service called Backordering, which will allow GoDaddy to grab the domain for you if the current owner does not renew it…meaning they do not pay their $10 for the year after it expires. Backordering generally only works if you know a domain is about to expire.
The best option you have is, find out who the current owner is and contact them directly by phone or email. To find the current owner, you can use:http://domains.whois.com/domain.php?acti…
Another option, is you can go through a domain agent. A domain agent is generally a person who buys and sells domains on a regular basis. I would be happy to help if you choose this option.
I must tell you, most domain/website owners are generally not interested in selling a site they already own. This results in the owner not emailing you back or asking for a high price, either way it never hurts to try.

Buy it from them. look at the who is and send them offer. or you can choose similar but different one.