I have no idea if the timeline between Shep's death and resurrection are vague, or if I'm just not remembering details, but it's still a 2-year span of time. Wouldn't necessarily be OOC for Liara to have taken a busman's holiday to Alchera to dig through the wreckage sometime after starting her Info Broker business.
Makes more sense for me if she did that between Shepard's death and arriving on Omega, though.
It's also possible that, if romanced, Liara got the tags from Hackett - but years ago after some official memorial service type thing - with the headcanon assumption that he was aware of the relationship. Then, if just friends, Hackett handed them over after Shepard's resurrection, figuring that Liara would see Shepard out in the Terminus first, and could return them to Shepard. So, either way, Liara could've gotten them from Hackett, but if she's an LI, she got them from Hackett so long ago that it doesn't rate a mention.
But, again - none of this is really spelled out. For all we know, Liara didn't even retrieve the armor part herself, but picked it up in some black market auction after scavengers went through the crash on Alchera and figured the famous Shepard's armor would bring in some bucks. Or, she skipped the auction, beat the crap out of the people who would do such a thing, and absconded with the armor. Huh ... actually, now I'm imaging that this is exactly what happened, except that's the job she hired Kasumi for, and how they know each other.
Oh, the drama. I'm going back to pondering their baby names. "Zakera Ramen-Shop T'Soni-Shepard", perhaps?