You should find them in the Site settings under Nintex Workflow - Workflow Templates.
http://< Site > /_layouts/NintexWorkflow/ManageWorkflowTemplates.aspx
Click on the name and you can see the Export link.
Hi Neil, there isn't a specific area for community built templates, at least at the moment. We are looking at having a specific area for that sometime in the future. In the meantime, you'll most likely find some workflow templates attached to blog posts in the Learn More or Tech Blog spaces. Here is a travel request one - Solution - Travel Request and Expense Approval.
And some others I quickly grabbed; a state machine template The Art of State Machine, or Create Parent/Child items using this Nintex forms/workflow with this trick. Hope that helps.