The two main places to find walkthroughs for casual games are
bigfishgames.com
gamezebo.com
Sometimes only one of them has the walkthrough for a game, and sometimes both have walkthroughs but one does a better job of describing a puzzle.
To find these walkthroughs, the fastest way is to use Google.
For a Penny Dreadfuls: Sweeney Todd walkthrough, you'd type these lines in the search:
Penny Dreadfuls: Sweeney Todd walkthrough site:gamezebo.com
Penny Dreadfuls: Sweeney Todd walkthrough site:bigfishgames.com
The site: part restricts the search to whatever site you have typed in (don't put a space between the : and the website or Google won't restrict the search properly).