Try ebay, someone else found some a month or so ago. There was a thread Look for 12 x 1.5 flat seat with washers, not acorn type lug nuts.
Did you find some? If not, here is a set: 20 TOYOTA OEM OE STOCK FACTORY WHEELS RIMS MAG LUG NUTS 12X1.5 1.5 21 HEX BLACK | eBay
I saw them last night. I used the compatibility tool and it stated it wasn't compatible. I sent them an email asking if they would work. thank you again!
They will fit, they are 12 x 1.5 which is what you're OEM lug nuts are. Moreover, they have the correct flat base and washer on them, same as OEM. Notice that the item is listed as compatible with a 2010 Prius, lug nuts haven't changed for your 2012.
