www.runningwarehouse.com is really good. They allow free returns and even include prepaid return labels in case you don't like them.
Ideally you would go to a knowledgeable running specialty store to get properly analyzed and fitted and then in the future order online, but that doesn't sound feasible right now. Lots of brands come in wide sizes, and some brands actually run wider than others in their standard sizing. For example I'm a wide in Asics shoes but Nike's Structure Triax fits me perfectly in the standard width, which kind of highlights the need to be fitted by someone at a specialty store. At least with Running Warehouse's return policy you can take your best guess and mail them back for free if you need to exchange them.