If you’re looking for the cheapest option it’s likely finding some sort of electronic tinkerer/enthusiast to take a look at the amp, just networking around with friends or something until you find someone who might be able to help. You could try and find a PC or phone repair shop, but I’d be slightly concerned about what they potentially might do depending on the place, I’d personally make sure you didn’t take it to a chain location as they’re both more likely to either not help or potentially screw it up (at least from my experience with computer repair places in the US, the chain stores typically suck ass).
You could get a new energizer, but if you did you might as well upgrade it, yes they are expensive for the good ones, but you will get a jump in sound quality that could still potentially make it worthwhile