Nectar

Nectar can be received as a room clear reward, from Charon's shop for 200 , as a rare extra reward for clearing Infernal Troves with a 5% chance, from the Fated List of Minor Prophecies, or in trades with the Wretched Broker.
Poseidon's boon Sunken Treasure also has a 10% chance of dropping a Nectar.
Dionysus' boon Premium Vintage will give Zagreus one free Nectar when chosen, and any Nectars found after that in the run (including the free Nectar) will raise .
With the Vintage Nectar upgrade from the House Contractor, Nectar will also give a random boon +1 Level when collected from a room reward. (As if Zagreus had picked up and used a Pom Slice.)
Fishing in Elysium is also a way of obtaining Nectar.