Portable speaker- battery pack problem
You haven't specified how much power the amp is supposed to put out, so apparently anything works.

Use as many 18650 lithium cells in series as you can without exceeding the amp's maximum allowed input voltage when the cells are at their maximum voltage.  The more tricky part is keeping track of the individual cells, and stopping before the lowest one gets to a voltage where it would be damaged.

Two car batteries in series might also work, as long as you don't charge them while connected to the amp.