Millions of domain names are already registered, so trying to come up with something original that nobody has is difficult at best.
You will probably have to buy one.
Look at http://www.tdnam.com, http://www.sedo.com, http://www.afternic.com for used domains, otherwise, you could always search for something generic that nobody has. Godaddy has a bulk domain search.
Here is a link to their bulk domain search (discount code included to get it for $7.99):
A 'bulk' domain search lets you search for 500 domains at a time.
You want a dot-com or dot-org. Anything else is a tough slog to brand.