You can easily locate and secure a webhost once you register a domain that you choose. You then export your site with iWeb and upload it to your webhost.
Shortening the url is a different matter, iweb more or less uses a long path to create the pages.
For instance I have mydomain.com/site name/pages.
When I use rapidweaver which provides nearly the same level of ease of use, but more flexibility. I basically have mydomain.com as my base page and then from there I have a Multi-tiered hierarchical website.
My point is you're kind of tied into how iWeb works once you get beyond the domain name portion of the url.