There are very few opportunities to get the missing fragments. Sometimes they are sold at the shop for real money, but that happens mostly short after the fragment event was, for people who missed some fragments. And once there where tournaments where you could get some fragments of passed events, but only few numbers, 5 or 10 or so, I don't remember.

So, basically if you missed the event, it's gone. You can still pull the champs from past events from shards, like other heroes. I got Kantra, who was an early fusion event champ, from an ancient shard.

Most  likely  there  will  be  "catch-up  events"  during  time  likes  Raids  birthday,  etc.  There  have  been  two  of  these  events  2date  so  far  and  you  can  get  like  15-25  fragments  for  legos  and  around  30  for  wpics  during  them.

https://hellhades.com/fragment-frenzy/

This was the most recent one in October where you were able to earn (up to) 15 fragments of Ruel/Versulf/Roric/Kreela/Yoshi/Brago/Bergoth/Drokgul/Elegaius.

Last March during the 2nd Anniversary events/tournaments we had opportunities to pick up fragments of Drokgul/Bergoth/Kreela, and I think I'm forgetting another one. 

Destroyer is correct about the amount of fragments you can expect to see pop up, so if you're short by more than 15-25 you could be waiting awhile to earn the remaining fragments strictly from tournaments or events.