Hmmm....
Just went on the Revellogram website and the kit is listed as being currently available:
http://www.revell-monogram.com/store/ModelDetail.cfm?Id=0&Number=5613&x=40&y=6
I'd contact them again and tell 'em they're full of it!
It may be that this kit is not in production at the moment, but it's not likely to be dropped from the catalog for any length of time, if at all. And if they can send you what they sent, they'll probably have more.
It's kinda lame to send a whole sprue of clear parts and not include one of the single most important pieces on that sprue.
Fade to Black...