The average apple employee that answers the phone is not going to look up part numbers.
It's not their job, and it's not in their training. (I'm not saying I like this, but understand it and learn how to work with it).
Make the genius bar appointment. Take in your HDD carrier and your serial number. Ask them if you can buy extra screws. They are technicians, not mindless drones. They can diagnose 'customer lost some screws' without seeing your Mac Pro.
But their options may be limited. They can't open a service request for parts without a serial number. That hdd screws may not be something that's in their system to order (but the carrier with screws probably is).
From the previous posters, it sounds like it is a $55 part for a new carrier with screws. An out of warranty repair is cheaper than a replacement, so I'm guessing that's why he was charged $18 and had to give his old sled.
Also... THEY ARE STANDARD HARD DRIVE SCREWS!!!!! Yes, the head isn't rounded and they include rubber washers, but they're not magic.