In the beginning they would take collateral from the people who found the animals in order to pay for the libations. However, people just brought the animals and ran away so they stopped taking collateral and started paying for it from the public monies.

Shikalim 7:6

R’ Shimon says the last mishna was one of the 7 takanos that bais din commanded. The others are:

A non-Jew who sent an olah from far away, if he sent the libations, we used his, if not the public paid for it.

If a proselyte donated a korban and died, if he gave the libations we use his, if not the public pays for it.

If a cohein gadol died, the libations for his mincha are brought from the public monies. R’ Shimon says that his inheritors pay for it and it was brought up whole, not ½ in them morning and ½ in the evening as normally brought.