N/A=Not available from this source. Curran Associates Inc. is a company specialising in the on-demand printing proceedings of many conferences. "Before IEEE partnered with Curran Associates for print on demand conference proceedings, IEEE titles would go out-of-print (OP) after three years. All proceedings from 2006 forward will remain available well into the future, but the older books are no longer available as complete sets." [www.proceedings.com/faq.html]
Rather order direct from the IEEE. See the first link below.
Otherwise get the digital version and print it yourself. See the second link below.
Yes, if it refers to a source then it may be Not Available as you have already mentioned. But when it refers to some information required then it may be not either Not Applicable or Not Available.
If speakers miss travel connections, get sick or go on "academic tourism" then the paper has not been presented, and cannot be published or cited. These are called "no shows". Also, if the presentation was a poster session, then there is usually no record that can be published.