A flumph is an intelligent creature. It should know a way.
This creature is perfectly capable of understanding that it will need an energy source. If it is following the party, then it must be aware of a solution.
It has an intelligence of 14, and is familiar with its own physiology, so it should be able to advise the party on where to find an energy source.
As the GM you’ll have to come up with a sidequest and invent an energy source. Perhaps the party has to take down a psyonic creature, or perhaps there is an object somewhere which the party can capture.
How might they feed their flumph
Its not a pet turtle. Its there because it wants to be, and its not their property. Flumphs are people too!