Pawn shops
Thrift stores
Online marketplaces (eBay, Craigslist, Facebook Marketplace)
Garage sales or estate sales
Local music stores that offer used instruments or equipment
Musical instrument consignment shops
Keep in mind that the type and variety of products vary based on individual shops and market conditions so it's a good idea to check your preferred sources on regular intervals or sign up for alerts regarding their new listings on websites. Before making a purchase always make sure to test the quality sound and inspect thoroughly the equipment that interests you.