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Module Resources
Unlike other OS CMSs, many of the best modules are paid rather than free, however there are loads of free modules as well. The cost is relatively low for most paid extensions and may be worth purchasing if it does the job and you don't want to code it yourself.
Drupal module review site. May be easier to find modules here than on the official site.
Tutorials
Of all three OS CMSs we are working with, Joomla module installation and configuration is the most non-intuitive to figure out. Hopefully this site will help a little with this.