The first thing you can try is to buy the domain name off of the person who presently owns it. This is not uncommon but it will cost you significantly more to buy an existing domain name than it would if you were to create one that no one currently has. However, asking if the domain is for sale from the current owner is one option you can explore.
Many people get caught up in using .com and have the impression that it is the only domain name available. This is not the case and you can try other domains such as .net, .tv, or even .org to see if you can still get the web address that you want. Of course the danger in using this route is that it can confuse customers and potentially cause problems with getting traffic to your website. The way to combat that is to make sure your website is optimized for exposure so that people will find your site when they search for it.
Sometimes patience is the best method to getting what you want and that may be the case with getting the web address you want. Check with the company that registered the domain name to the current owner and then see when their registration expires. Many registration companies allow you to put in a back order which means that if the current owner decides to not update their registration, or forgets to, then you will be able to purchase the domain name.
Another method you can try is to use a slight variation of the name you wanted and see if that variation is available. For example, if you wanted to register ABCcompany(dot)com but someone else has already registered it, then try TheABCcompany(dot)com or some other variation on the web address. The potential for confusion is there again but with a proper search engine optimization approach, you can eliminate much of the confusion that your address could cause any potential customers.