Screws required to attach HD's to mounting tray for Apple Mac Pro Tower Mid-2012
I assume you are talking about the first generation MacPro "CheeseGrater" tower. for future reference it is best to include the model number with your question so there is no confusion.

The screws are actually standard threads and common to ANY hard drive. You should be able to get these screws at any hardware store worth its name. Just take the HD with you to make sure you get the right ones. 

The rubber grommets can also be sourced at a hardware or electrical parts store. Just get some grommets that will fit in the holes in the Apple HD trays and then size the screws long enough to span the gap between the grommet, the HD tray and into the drive. Depending on the size of the screw head you may also need some flat washers.

I did this with my MacPro 1,1 many years ago and it worked a treat.

Sourcing the Apple genuine parts proved frustrating and the few places/people that carried them wanted way more money than it was worth. In total you should be able to get screws, grommets and washers for well under $10