Any old 288-pin DDR4 ECC DIMMs will work in the iMac Pro.
Non-ECC DIMMs should also work, be cheaper, and potentially also be faster, but you forfeit any benefits of error-correction if you mix the two.
Additionally, without testing, there is no way to tell whether or not Apple has built the iMac Pro to only accept ECC RAM.
Here is the teardown guide for the iMac Pro (Step seven is where they discuss RAM): [guide|101807]
Also, here's a good article about ECC vs. non-ECC RAM from Tom's Hardware: https://www.tomshardware.com/reviews/ecc-memory-ram-glossary-definition,6013.html