There is no way to get it now.
It was announced that it was tied to the event and would be limited to that event as a special item to get.
Adding onto what was said above:
The Buster Sword R5 (unlike the Takeover’s Buster Sword R2R5) is a Universal Mythic Melee Cosmetic that has no gameplay abilities and can be equipped in place of your heirloom or fists. The sword will not be coming to the Mythic Shop after the event ends, so unlock it while you can!
From: [EA] APEX LEGENDS™ & FINAL FANTASY™ VII REBIRTH EVENT
IMO it does suck, does create a lot of FOMO and well, just not all too great.
The Buster Sword isn't all that special either, as it doesn't have a lot of animations as far I'm aware, plus it takes up a large portion of the screen.
So I don't think you'd be missing out on much.
There was likely an agreement with final fantasy on why it couldn't be in the mythic shop. Plus because it is universal its a bit better than others so even if they could my guess is the cost would not be the normal mythic shards. The reason being is once you have a universal one it kind of kills the desire to get one for another legend.
As to whether the sword is a good heirloom vs others, I disagree with xhelperz. I own quite a few heirlooms and it's pretty nice. On all heirlooms the inspection animations are a cool maybe look at them once thing. What matters for me is the look and how the heirloom reacts during normal carrying and pulling. The sword has great animations in tha regard.
