Greenwood's 1827 map of London. Very detailed, you can select particular sections to examine. This section includes the general area of Fagin's gang, and (if you look in the corner, the northwest corner of Pentonville. Check out the full map here.
Specific to Oliver Twist
A really brilliant source is The Guardian's interactive Oliver Twist walking tour. An hour-long podcast leads the way down the path Oliver and the Dodger might have taken through London, stopping to talk about important places on the way. In addition to the Google Map, there is a lovely illustrated map of the walking tour that you can find here.