I doubt there are any originals. There are no originals of any of the books of the Bible or just about anything that old. All we have are copies of copies, though some of those copies are pretty old themselves.
However, we have more than quotes from Eusebius. We do have entire manuscripts of many of the early church writings. Where all we have is quotes, the quotes tend to be compiled together as "fragments of Papias" or something like that. A book like the letter to Diognetus will come from various manuscripts, maybe only three or four, of various quality, but it is not just pieced together from quotes in other writings.
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