These days it’s pretty hard to find any ponies out in the wild. Not impossible (ex. People do get lucky and it depends on where you live) but I highly doubt that a Mimic is just floating around out there without someone realizing what she’s worth. Personally I go thrifting all the time and I have never found any non-G4 pony “in the wild” besides in specifically curated antique malls and vintage toy shops. I don’t think Mimic is inherently more rare than similar ponies but she’s highly coveted as a sort of status symbol, and also just a pony many people really like, so she gets snatched up quickly.
Overall, I think with finding ponies “in the wild” already being tough and non-collectors becoming more aware of the prices collectors will pay, finding her for a reasonable price in person or online would be an insane stroke of luck.
So… don’t give up, but you’ve created a legendary quest for yourself pony friend!