I have the same problem. A peeled off tire hit my front bumper really hard a few months ago. The only damage was the lost washer cap. The best place to find them is at your dealer. They maycharge you about $30 for it which is a joke. But it beats driving a nice car with a big hole in the bumper (except if the car is black

). There are some sellers on ebay sometimes but i just missed one...
Also, make sure that the washer itself is not damaged! If something hit the bumper hard, it might be broken. It is easy to see if you wash your windshield with the headlights on because the liquid will go in different directions. You may need 2 people for this because the headlight washer only opens up for 2 seconds.