During a fight, you perform a "Dragon Ball Hunt." Landing a Super Attack, losing a character, or performing a combo over 20 hits grants you a Dragon Ball. You can collect all seven in a single match (rare) or across multiple matches.
On PC (Steam), mods like exist on GameBanana. These force-unlock DLC characters locally for offline play, but they do not transfer to online matches and can cause desyncs. Most mods simply edit the game’s save file. Saiyan Dragon Goku Fighter Z Unlock All Characters
Note: You cannot buy DLC characters with Zenny. During a fight, you perform a "Dragon Ball Hunt