So my sister tore one of her mudflaps and needed some hardware to re-install it as all the hardware broke except the special metal bracket for the Imprezas.
After a bit of researching, gave up on getting the hardware from Rally Armor and didn't want to use the metal clips found at the hardware store. I'd rather have the hardware break than having the bumper torn. Did some more researching and found they're called u-nut. From there, found that the exact one supplied with the flaps are Toyota u nut #14.
I ordered a bag from Ebay from a guy in Hong Kong since the shipping was only 2$. All in all it came to 8$ for 25 of them. I just received mine today and it's the exact same part except it took 1 month to arrive
I hope this will be helpful for you guys.