Remember if you are going to look for the original install disks you must make absolutely certain that the original Apple
install disks are for your exact MacPro 5.1, yours being Mac Pro. 5,1 Quad-Core Intel Xeon 2.8GHz
as Grant Bennet Alder has echoed.
Any other MacPro 5.1 Apple install disk will not do it has to be for your specific model of Mac Pro.
So read everything twice to confirm before committing to a purchase.
Are you sure Apple said there was no disk for you to find, the OS does come pre installed but you always got the 2 DVDs
with it, Disk 1 would be the Snow Leopard 10.6.4 installer and Disk 2 would have been the Apple Hardware Test.
I would have another look round, do you still have the original box, it might be in there.