This number will vary by geography. Are you asking about Newark, specifically? The percentage will also vary according to what list source you are using.
If you are asking about published lists such as Realty Trac, the information that appears on their lists (and fed to other sources from their list) is gathered from public filings at various courthouses across the country. If you are asking about how many filings there were in Newark, the information could be obtained at the local courthouse.
In NJ, all foreclosure actions are filed in Trenton, and assigned a docket number in sequential order. All foreclosure filings are easily identified with "F" preceeding the docket number. Some cases are transferred to the court of local jurisdiction; others reach final judgement in the Trenton foreclosure unit or are resolved and never leave the Trenton foreclosure unit. The number of cases actually transferred to a jurisdiction will not reflect all the filings.
I don't know of a particular source that will give you a town by town report. You can get state data at a number of sites. My reference to these sites is not an endorsement of the services or products that they sell.