This is simple. No. You don't get a fee, not off the top or otherwise. What makes you think you get a 15% fee> Did you have a contract with every heir in which they all agreed to pay this fee? No, of course you didn't.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
If you have a contract, even a verbal agreement, with one of the heirs to perform the search, then you may be able to recover from that person. But you will not be able to get anything from the people who were not part of the deal you made.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.