Hmmm. Good point. There are probably a few reasons why individual issues aren't listed, however I'll make the suggestion. Perhaps at least there could be a section saying that past issues are available and how to go about ordering them.
Meanwhile, just e-mail the NSS Office to request specific copies or to see what is available.
nss@caves.org.
If the issues you want are not available in the office, you might ask on this forum if cavers have copies to sell or pass on.