After some thought - and concluding that even the replacement flaps will also eventually deteriorate and break - my suggestion is that you install "anti gust hood covers". These covers are metal / aluminium so they will last and not rust. They can be installed directly over the existing vent. They also block wind getting into the vent duct and stop the flaps making noise in the wind. They also have a screen to prevent the entry of vermin into the cover.
Because you have a stucco wall and because the stucco is essentially styrofoam with a layer of finish over it, the screws used to install the cover will need to be about 3.5 inches long to get into some structure.
It looks like the model - VT0048 3-1/4" x 10" Anti-Gust Hood WHT - is your best option. Might as well do the powder room vent at the same time.
http://www.imperialgroup.ca/generalventing.cfm?c=227Note - that these covers should not be installed over dryer exhaust vents.