http://www.iso-parts.com
The website shown above will search NSNs, but in many cases the descriptions aren't much better than what is on the packages. There are other online NSN search engines available as well.
I have an old set of supply microfiche from the 80s which I use for these older part numbers. The number you have given is for the hammer spring assembly on the Canadian FN rifles, all versions.
It is rare to see NSNs from the 1005 class these days, the military will throw them out or destroy them verses selling them.