The NGA Collage Machine allows you to exercise your collage making skills without finding your own images.  It could be a useful starting point on your collage making journey.

Create an interactive digital collage using VUVOX.

If you have an android device you can download this app from Adobe that will help you create collages.  You can upload your basic creations to their cloud and then download them into Photoshop for refinement.

Learn from viewing the work of professional collage artists.

Alter your images before using them in collages. 

Upload your artwork and alter with various 'pop art' style filters.  Use these for collage ideas.