The best way to do this is to use puppetdb, which you can then query from manifests using the puppetdb_query() function.
Your query would look like (untested but i think this is right...)
$nodes_array = puppetdb_query('inventory[certname] { resources { type = "OurSystem" } }')
(You may see references to "exported resources" which accomplishes a similar goal but IMO querying is better because it has a superset of exported resource functionality and you don't need to know beforehand which resources you want to mark as being 'collectable'.)
HTH
--eric0