Google Scholar, which indexes material found on scholarly sites, including journal websites, also provides citation counts and lets you search by cited source. Google Scholar indexes the bibliographies for a larger but also less defined set of sources, including some books scanned by Google, and newer articles, than Web of Science and Knowledge.
Google Scholar can be found on the web at http://scholar.google.com.
To do that, look up the article or book you are interested in finding citations to.
You can do a basic search or try the more precise Advanced Scholar Search.
Currently, you can choose Advanced Scholar Search from the drop-down triangle on the main page of Google Scholar
This will pop up a fill in window for the advanced search.
To search for a specific article,
- put the Author's Last name in the box marked "Return articles authored by" and
- put the journal title, if there is one, in the "Return articles published in"
You can also be more specific by:
- Putting title words in the "with all of the words" box and changing 'where my words occur' to in the title of the article
- Entering a date in the "Return articles dated between" boxes.
Among the results that come up, will probably be one or more for the cited source you're interested in.
In the lower left corner of each entry will be the indication "Cited by [number]".
Clicking on the Cited by link brings up the list.
You can opt to enter more keywords here and check Search within citing articles to narrow down your results.
Creating an Alert
Whenever doing a search in Google Scholar, you will be offered the option to create an alert where Google will email you if more items matching your search are retrieved later. Look for the 
option in the lower left column of your search results.
Clicking on brings up a new page:
The "Alert query" shows you what you are searching for.
Enter your email address and click "Create Alert".
Google will email you when more material is indexed that matches your search.