Why not just ask if they have the product? If they don't, just leave. They're sellers, not historians. Would you expect a kid working at the computer section of Best Buy to know the history of Intel chipsets?
I can't speak for Neiman Marcus as I'm Canadian but heading to Sears, the Bay, Holt Renfrew or Shoppers around here would yield an experience similar to your own. I don't blame them though, as the majority of their customers aren't frag-heads and likely don't care at what time of the day Tom Ford brushes his teeth daily.