I lost one of my wheel covers a couple of months ago and was able to find a used replacement easily on eBay. It had a couple of scratches (so it was like all the other wheel covers on the car) and cost about half of the price of ordering one from the dealer's parts department. Log into eBay and search for "272861992164" which is the item number of a wheel cover which may fit your needs. The price is $28.99 including shipping. (Just make sure it's what you want and that your comfortable with the seller and eBay sales process.) I also found this web site "ToyotaRecyclers.com" which I know nothing about but may be familiar to others in this forum.
Remove other wheel cover from the side of the lost one. Wheels on one side, wheel covers on the other, PLUS a spare cover before you take Mendel's advice.
2 options: 1. remove the other hubcaps and sell them on ebay ,and buy 4 center hubcaps (56mm?) to cover the center piece. 2. buy a used one from ebay for around $40, or but from dealership for around $60. (This is what I did, I bought from ebay for about $35 and I've also zip tie them at the back for the hubcap to the wheels to avoid future losses. The only down side is I need to remove the zip tie each time before tire rotations, then need to zip tie them again after rotation. Personally I like option#1 because it looks good and little work. But my wife likes the 2nd options because she thinks the factory hubcap looks better, and she doesn't feel the pain of doing the zip tie as I'm the one who is doing it.
I've noticed a few Prius with zip-tied wheel covers. Honda has (or had?) a good wheel cover system: the lug nuts had plastic rings on the shoulders, and the wheel covers had openings for the wheel studs, plus flat zone within those opening that the lug nut's plastic rings locked against. Very secure, and no scratching of the rims by the wheel covers. Pic of the lug nuts:
What's the Prius C hub cab looks like? I have a set for sell but it's from a 2017 Prius Prime...should fit all 15" wheel though. For Sale - 2017 Prius Prime wheel covers / hub cabs | PriusChat