No they shouldn't require part numbers. Not everybody will be able to find part numbers. That's the job of the person on the parts desk! You tell them what it is, and they look through the system to locate it.
Sounds like you just have an arsehole parts desk person! Saying that, all the Ford ones I've been in have been arseholes. All the Nissan ones I've been in have been uber friendly. I've even gone behind the desk into the warehouse part a few times (that may be because I've been there enough fekkin times to be on first name terms

).
Just go and ask. If the person is arsey, be arsey with them and ask for obscure things too. Like the screw that holds the centre console in

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"Ahhhh crap... Just a bit too over eager really!""I used to be an adventurer like you, but then I took an arrow in the knee"