It's a common problem, especially if you lower the car.
I've been quoted up to $300 a set but I got my replacements from MARK GIBBS PANEL HOUSE, 45 Randall St Slacks Creek 4127, (07) 3808 8737 for $95 each which includes all the plastic lug/screws.
I've got a similar problem on the left front., cept mine is hanging off a bit and hits the tyre when I'm turning. Nothing too drastic but the noise does get annoying sometimes.
It's a rather large, odd-shaped bit of plastic but you could probably make something close to similar if you had an original to use as a template. You may run into a little bit of trouble with the molded areas though.
I picked up a replacement from my local Mitsubishi dealer (Nunawading Mitsubishi in Melbourne). They had it in stock also. Total cost for one guard including labour was about $200.
The reason I got it replaced was because I noticed stones getting through and hitting the rear of the bottom headlight.