Registered where?
If you cannot find it on the public records-it was not registered if you did an adequate
search.
If a deed was prepared and filed-the attorney's name might be on the document as the preparer.
The attorney might have a copy of the trust.
You could also advertise in the local county Bar Association.
Unlike land records, there isn't a place where revocable trusts are all recorded and made a matter of public record. In fact, one of the reasons that people choose a living trust is to preserve privacy. If you see evidence in the land records that real estate was transferred into the trust (keeping in mind that real estate transfers are a matter of public record if they are recorded), the attorney that prepared the deed may also have prepared the trust. Other than that, your best bet is to search your father's belongings, ask attorneys he may have used, and possibly inquire through the local bar association.
As the others attorneys have stated, there isn't a trust registry to look up trusts. It is possible that the trust was recorded, most typically aren't. You can check the county clerks office, recording department, to see if a trust was recorded. Your other option is to contact the local bar association, in the area your father lived when the trust was drafted, ask them to email attorney's to see if you can find the trust. I often get emails from my local bar associations asking similar questions.
Good Luck