I need to find a flow cell, either in Stainless Steel or PEEK, capable of supporting a pH sensor and a DO probe. Prefer that it be COTS, from a bioprocess supplier.

While there are a host of bioprocess suppliers who provide flow cells, most all of which being commercial off the shelf (COTS), the sensors you mention needing to support made all the difference. I would recommend you take a look at the Hamilton offering of flow cells. I have provided a link where you can learn more.

Hamilton Flow Cells