Mercy is my girl, and I love nearly every single costume design she has -- especially her default. Unfortunately, it's also a hell of a costume to make up on your own, and it's nothing I'm ready to attempt at my current level of no skill whatsoever.
For those of you sitting squarely in my boat but with a great deal more disposable income, there's this full Mercy costume (no gun/Caduceus staff) selling for a whopping $1,212 on Ali Express.
This comes customized to your measurements, but plan well in advance since the seller warns that it will take 2-3 months to produce. And that's not even counting the shipping time.