Drop .chef/plugins/knife/source_ingredient_chef_client.rb into your .chef/plugins/knife directory and run:
knife source ingredient chef client
:file_cache_path and :data_bag_path must writable and might need to be set in your knife.rb
The latest available versions of chef_client will be downloaded into your file_cache_path and your data bag path will get a chef directory created which will be populated with data bag items for each version of chef that is available.