Besides the PO Board, which I'm sure can be very helpful, you will have to set your criteria, so why not go to some well established provider sites as well, and well established agencies and read the criteria they have, and see what fits you. You can see lists of well established providers as they are reviewed on TER in your locale or others, and look at their websites for their criteria.
This would also allow you to call some of them and ask them, or PM them here.
And as Mr. Fisher stressed, that a perspective client is a TER member is not a way of verifying them. TER doesn't have any verification criteria for joining. Anyone can join TER.
Many providers require one of the verification services you mentioned, and may require additional references.