There have been, I believe, three different Fragment Top Up events over the past couple years. This is where they pick a half dozen fusions or so and have special Dragon Tourneys, or Champ Training Events, etc, for you to earn small amounts of fragments for previous fragment fusion champs.
I think the most I have seen available was 20 or 25 per champ, and that is IF you did the events AND *won* the tourneys. And I think in at least one of those times they sold frags at the end to finish out champs if you wanted?
Anyways, I have used this process to finish up a fusion I didn't finish from a while ago by dropping a decent amount of gems to secure some tourney wins. :D