Your local LBS or Co-op should be able to source a 2nd hand one? If not there should be someone on here that should have a spare?
Rear gaurds can be modified to fit. Also the MTB boys can fit splash guards on frame, they attach to the down tube via Velcro strips or cable ties.
Failing all of these eBay should have some cheaper ones available.
I with a rear rack am running a full set, on some bikes you might need to file a groove to clear interference from rack.
Large plastic bottles cut in half down the bottle with cable ties to frame make a good make shift guard too.

Check out www.bikehacks.com there are some great ideas on here for emergency type situations.