I paid a developer to modify my app that I bought, however he would not send me a copy, is this common?
Is there a way I can down load all the files it to my home computer as a back up?
ALSO how do I create a link to the app, so I can put it on my website.
Unless you had some agreement where the developer owned the rights to the source code, I would think no, it is not common. You should have the full Xcode project including source code.
Apple does not store anything except the generated binary so there's nowhere to download anything from except getting it from your developer.
The App Store resources page has everything you need to know about marketing an app.