This is not Apple. We are users just like yourself. If you need a power cable for your Mac Pro I suggest you do some Googling for it and also look on eBay. You will need to know what type of internal connection you would use in the Mac Pro. For example, there are supposed to be a couple of SATA ports available on the motherboard.
There are Y-adaptors/splitters that can connect to a SATA connection and provide two new connections. You may have to do some searching as these are sort of specialized adaptors.
Not true. There are SATA connectors that carry both data and power. There are motherboards that use separate power and data cables for SATA drives. Your question stated:
Please let me know and if you have the cable I would need to purchase.
This clearly is a request for someone here to see if there is a cable we could sell to you. That would imply you are asking Apple the question. It's not my fault that you were not clear in what you asked.
Nevertheless, my response is just as applicable to one form of cable as to another. If you have no available power connection in the computer, then you need to split one that you are currently using. This could either be the SATA power connector or it could be a Molex power connector as used on older SCSI or ATA drives.