Are there elisa testing centers that will accept plates coated with antigens purchased from another company, and then provided to them from me?

Doubtful: Since it is impossible for the testing company to know for sure what they are dealing with, it is unlikely that they will accept your sample. Even if they would, the cost in customization will likely be prohibitive.

No clue.: You will have to call them and ask. This is not something that is normally done and so there is no professional buzz about what happens in these cases. I would expect that there might be some concerns about the purposes of such a test, and the quality and method of collection of the sample, as well as issues of pt confidentiality and consent that would be a barrier to having this done.