I'm sorry but you can't.

There have been special events that award old fragments before, and sometimes I think they would show up in the store.

Unfortunately you are most likely sol with that.

October is usually when Plarium does their "Fragment Catchup Events" where you can earn the last 25 of a set of frags for a select group from the previous year. I have completed a fusion I missed that way myself. But, two problems... it is a LONG way off, and while they have done it a couple times now, no gaurantee it will happen this time.

Sorry I don't have a better or easier way to do it. We suggest "Universal Fragments" as a reward occasionally, but I am not sure Plarium wants to go in that direction. even if they were QUITE rare.