If you use:
Website as Book, (Electronic Publication)
Online Publication (Image Copy by Author)
Online Publication (Image Copy by Title
you can add a url. The URL is already built in to the source and is, in effect, the repository.
You can also add the url in the entry box for a citation url for all the citations that reference this book - especially if each page has a different url.
Or, as already suggested, you can use the generic source template and select a repository.