I wouldn't personally suggest a hyphen, can be very confusing and you really do want the URL to be as clear as possible. Of course your URL doesn't have to be your actual store's name, remember that. You could have a pet store called Mud Ducks Incorporated, and then a URL MDI.com for example.
The point being that you shouldn't get too tied up on limiting a business name because you can't get the duplicate URL. Another option is to consider different domain extensions, .net, .co, .biz and so on. However, if you do that, then be really careful about who does have the popular choices, such as .com or .com.au, because if people do make a mistake and enter the wrong extension by mistake, then you will be directing business to your competition.
Last idea, if you do find a name that is locked up you can try and flip a letter in the word maybe. So if there is an F in the word, maybe flip it for a PH, then really highlight the PH and turn it into a logo type of thing.