For many years, I used PCI cards with a Silicon Image 3124 or 3132 SATA controller in my Mac Pro.
They are essentially the same. The 3132 is PCIe, and the other one is PCI-X.
You can purchase them from Addonics, who will supply a Mac driver for you. Newegg and Amazon work too.
These are simple controllers. I did run into a problem with them when I was running a bigger zpool under Solaris 11, it would not keep up with the load. I use an 8-port LSI controller for that now in a Linux server. I no longer have the old Mac Pro so I don't know how it might work for Mac ZFS.
This intel-rebadged LSI card is ideal for personal use, small ZFS servers. In addition to the controller, you will need two SFF-to-SATA cables, about $20 each.
But you can start out small with a $20 Syba card.
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