Thunderbolt PCIe for 2008 Mac Pro

There are not such cards, and as long as Intel controls the chips required to implement ThunderBolt, there never will be. You did not miss anything. They do not exist.

To use ThunderBolt, a computer must have left the factory "ThunderBolt-ready". There are few such computer made by Acer -- but these are just ThunderBolt computers where the ThunderBolt Hardware was placed on an optional daughter card, then was made optional to save money.

I wish to concur with Grant that your Mac, just like my Mac, will never have ThunderBolt.

I would interested in why you think you might need ThunderBolt.

What I did in my 2010 Mac Pro was to install a card to implement USB 3 which opened up access to a number of devices for me.

You can also install E-Sata cards, since you explicitly mention external drives.