StackOverflow -- Search and ask questions about software installation, operation and troubleshooting. Be sure to tag your question with "cytoscape" so we can find it!
BioStars -- Search and ask questions about data anaysis and visualization, including recommended apps, databases and workflows. Be sure to tag your question with "cytoscape" so we can find it!
Social networking services are good data sources. You can write simple web crawler using scripting languages and save it as standard data format such as GML, GraphML, or simple tab-delimited text. Cytoscape supports many standard graph file formats.
Check out the other community resources for Cytoscape.