Music format for bringing to audition/demo

Had my system for 20+ years, time for an upgrade. I have not been in a hifi shop since 2000 and at that time I would simply bring CDs. Nowadays is it most common for people to bring digital music on a thumb drive or some other way and what is that? Or do people rely on dealers having nice streamers and they just look up what you want to hear? 

Basically I want to know what it most common and how I can bring music that say I downloaded from hd tracks or similar sites to the dealer.